feat(rpc): implement getrawmempool RPC method

[oxarbitrage]

Motivation

Will close #3160 eventually when done.

Solution

Implement getrawmempool

Review

@jvff already is working with me in this so will be nice to get a review from someone else.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[oxarbitrage]

By the lightwalletd integration tests logs it seems the getrawmempool does not provides any output:

Mar 11 19:36:23.183  INFO {net="Main"}: zebra_rpc::server: Opened RPC endpoint at 127.0.0.1:55578
{"app":"lightwalletd","buildDate":"","buildUser":"","gitCommit":"","level":"info","msg":"Starting gRPC server version v0.0.0.0-dev on 127.0.0.1:0","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"warning","msg":"Starting insecure no-TLS (plaintext) server","time":"2022-03-11T19:36:23Z"}
Starting insecure server
{"app":"lightwalletd","level":"info","msg":"Got sapling height 0 block height 0 chain TODO: main branchID ","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"info","msg":"Found 0 blocks in cache","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"error","msg":"Couldn't open file /tmp/zebrad_testsAMA4SB/db/TODO: main/prices for writing: open /tmp/zebrad_testsAMA4SB/db/TODO: main/prices: no such file or directory","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"error","msg":"Couldn't read historical prices, starting with empty map","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","error":"error requesting block: 0: Block not found","height":0,"level":"warning","msg":"error zcashd getblock rpc","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"info","method":"CurrentPrice","msg":"Service","price":154.295,"provider":"github.com/adityapk00/lightwalletd/common.fetchCoinbasePrice","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"info","method":"CurrentPrice","msg":"Service","price":154.458729947906,"provider":"github.com/adityapk00/lightwalletd/common.fetchCoinCapPrice","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"info","method":"CurrentPrice","msg":"Service","price":154.5,"provider":"github.com/adityapk00/lightwalletd/common.fetchBinancePrice","time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"info","method":"SetCurrentPrice","msg":"Service","price":154.458729947906,"time":"2022-03-11T19:36:23Z"}
{"app":"lightwalletd","level":"info","msg":"Ingestor adding block to cache: 1","time":"2022-03-11T19:36:33Z"}
{"app":"lightwalletd","error":"error requesting block: 0: Block not found","height":801,"level":"warning","msg":"error zcashd getblock rpc","time":"2022-03-11T19:36:33Z"}
{"app":"lightwalletd","level":"info","msg":"Block hash changed, clearing mempool clients","time":"2022-03-11T19:36:35Z"}
{"app":"lightwalletd","level":"info","msg":"Ingestor adding block to cache: 801","time":"2022-03-11T19:36:43Z"}
Error: 
   0: stdout of command did not contain any matches for the given regex

Location:
   /app/zebra-test/src/command.rs:330

Command:
   "lightwalletd" "--zcash-conf-path" "/tmp/zebrad_testsAMA4SB/lightwalletd-zcash.conf" "--data-dir" "/tmp/zebrad_testsAMA4SB" "--log-file" "/dev/stdout" "--grpc-bind-addr" "127.0.0.1:0" "--http-bind-addr" "127.0.0.1:0" "--no-tls-very-insecure"

Exit Status:
   command terminated unsuccessfully by signal 9

Match Regex:
   "(Mempool refresh error: -32601: Method not found)|(Another refresh in progress, returning)"

See https://github.com/ZcashFoundation/zebra/runs/5516017007?check_suite_focus=true#step:4:220

[jvff]

I pushed some commits with the suggestions, but feel free to revert or discard them if you want to do it differently :)

[codecov]

Codecov Report

Merging #3851 (a39e92e) into main (4197704) will decrease coverage by 0.07%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main    #3851      +/-   ##
==========================================
- Coverage   78.82%   78.74%   -0.08%     
==========================================
  Files         295      295              
  Lines       33764    33788      +24     
==========================================
- Hits        26613    26606       -7     
- Misses       7151     7182      +31