importer simultaneously importing a large number of data files causes one layer to exit

[dbacyj]

A total of more than 500 files，The error is as follows：

[yixinglu]

seems related to #95

@veezhang could you help to debug this?

[veezhang]

seems related to #95

@veezhang could you help to debug this?

👌

[veezhang]

panic: send on closed channel

goroutine 241 [running]:
github.com/vesoft-inc/nebula-importer/pkg/reader.(*Batch).requestClient(0x400071a200)
        /home/runner/work/nebula-importer/nebula-importer/pkg/reader/batch.go:66 +0x128
github.com/vesoft-inc/nebula-importer/pkg/reader.(*Batch).Add(0x400071a200, 0x1, 0x40b0caeb60, 0x2, 0x2, 0x25)
        /home/runner/work/nebula-importer/nebula-importer/pkg/reader/batch.go:36 +0xc8
github.com/vesoft-inc/nebula-importer/pkg/reader.(*FileReader).Read(0x40001bc050, 0x0, 0x0, 0x0)
        /home/runner/work/nebula-importer/nebula-importer/pkg/reader/reader.go:166 +0x504
github.com/vesoft-inc/nebula-importer/pkg/cmd.(*Runner).Run.func2(0x40002998f0, 0x4000331080, 0x40001bc050, 0x40002001b0, 0x26)
        /home/runner/work/nebula-importer/nebula-importer/pkg/cmd/runner.go:74 +0x28
created by github.com/vesoft-inc/nebula-importer/pkg/cmd.(*Runner).Run
        /home/runner/work/nebula-importer/nebula-importer/pkg/cmd/runner.go:73 +0x5bc