Add es:migrate command (#81)

## What does this PR do?
Add a new command to the `es` module to migrate data and mappings from an Elasticsearch server to another using bulk API:

![kourou](https://user-images.githubusercontent.com/7868838/98674476-25e8b000-2359-11eb-9b0e-e020d54505f3.gif)

src/commands/es/migrate.ts

@@ -0,0 +1,135 @@
+import { flags } from '@oclif/command'
+import { Client } from '@elastic/elasticsearch'
+import cli from 'cli-ux'
+import chalk from 'chalk'
+import emoji from 'node-emoji'
+
+import { Kommand } from '../../common'
+
+export default class EsMigrate extends Kommand {
+    static initSdk = false
+
+    static description = 'Migrate all the index from one Elasticsearch server to another'
+
+    static flags = {
+        help: flags.help(),
+        src: flags.string({
+            description: 'Source Elasticsearch server URL',
+            env: 'KUZZLE_ES_SRC',
+            required: true
+        }),
+        dest: flags.string({
+            description: 'Destination Elasticsearch server URL',
+            env: 'KUZZLE_ES_DEST',
+            required: true
+        }),
+        reset: flags.boolean({
+            description: 'Reset destination Elasticsearch server',
+            default: false,
+        }),
+        'batch-size': flags.integer({
+            description: 'How many documents to move in batch per operation',
+            default: 1000,
+        }),
+        'no-interactive': flags.boolean({
+            description: 'Skip confirmation interactive prompts (perfect for scripting)',
+            default: false,
+        }),
+    }
+
+    static examples = [
+        'kourou es:migrate --src http://elasticsearch:9200 --dest http://otherElasticsearch:9200 --reset --limit 2000',
+        'kourou es:migrate --src http://elasticsearch:9200 --dest http://otherElasticsearch:9200 --reset --limit 2000 --no-interactive'
+    ]
+
+    private src: any
+
+    private dest: any
+
+    private async migrateMappings(index: string) {
+        const { body } = await this.src.indices.getMapping({ index, format: 'json' })
+        const mappings = body[index]
+        await this.dest.indices.create({ index, body: mappings })
+    }
+
+    private async migrateData(index: string, batch_size: number) {
+        const queue = []
+        const { body } = await this.src.search({
+            index,
+            scroll: '20s',
+            size: batch_size,
+            body: {
+                query: {
+                    match_all: {}
+                }
+            }
+        })
+        const scrollId = body._scroll_id
+        const total = body.hits.total.value
+
+        if (total === 0) {
+            return 0
+        }
+
+        const progressBar = cli.progress({ format: chalk.blue(' [*] Importing |{bar}| {percentage}% || {value}/{total} documents') })
+        progressBar.start(total, 0)
+
+        let count = 0
+        queue.push(body.hits)
+        while (queue.length && total !== count) {
+            const { hits: docs } = queue.pop()
+            const bulk = []
+            for (const doc of docs) {
+                bulk.push({ index: { _index: index, _id: doc._id } })
+                bulk.push(doc._source)
+                count++
+            }
+
+            await this.dest.bulk({ body: bulk })
+
+            progressBar.update(count)
+
+            const { body: { hits } } = await this.src.scroll({
+                scrollId,
+                scroll: '20s'
+            })
+            queue.push(hits)
+        }
+        progressBar.stop()
+
+        return total
+    }
+
+    async runSafe() {
+        this.src = new Client({ node: this.flags.src })
+        this.dest = new Client({ node: this.flags.dest })
+
+        if (this.flags.reset && !this.flags['no-interactive']) {
+            this.log(chalk.red(`${emoji.get('fire')} Are you sure you want to reset ${chalk.bold(this.flags.dest)}?`))
+            await cli.confirm(chalk.redBright(` ${emoji.get('fire')} You will lose all the data stored in it (Type "yes" to confirm)`))
+            await this.dest.indices.delete({ index: '_all' })
+        }
+
+        this.logInfo(`Fetching indices list from ${chalk.bold(this.flags.src)}`)
+        const { body } = await this.src.cat.indices({ format: 'json' })
+        const indices = body
+            .map(({ index }: { index: string }) => index)
+            .sort()
+        this.logOk(`Found ${chalk.bold(indices.length)} indices in the source Elasticsearch server\n`)
+
+        this.logInfo(`Starting indices migration from ${chalk.bold(this.flags.src)} to ${chalk.bold(this.flags.dest)}`)
+        for (const index of indices) {
+            this.logInfo(`Importing ${chalk.bold(index)} to ${chalk.bold(this.flags.dest)}`)
+
+            await this.migrateMappings(index)
+            this.logOk('Mappings successfully imported!')
+
+            const count = await this.migrateData(index, this.flags['batch-size'])
+            if (count === 0) {
+                this.logInfo('No documents to import\n')
+            } else {
+                this.logOk(`${chalk.bold(count)} document(s) imported!\n`)
+            }
+        }
+    }
+}