Leveled event pool

[vadimalekseev]

Closes #688

Before (best of 5):

$ export POOL_LOW_MEMORY=false && go test -bench "BenchmarkLightJsonReadPar" -run ^$ ./plugin/input/file

goos: darwin
goarch: arm64
pkg: github.com/ozontech/file.d/plugin/input/file
BenchmarkLightJsonReadPar-8            1        2235307292 ns/op         649.23 MB/s      224088 B/op       2359 allocs/op
--- BENCH: BenchmarkLightJsonReadPar-8
    file_test.go:1187: Memory used: 249MiB

After (best of 5):

$ export POOL_LOW_MEMORY=true && go test -bench "BenchmarkLightJsonReadPar" -run ^$ ./plugin/input/file

goos: darwin
goarch: arm64
pkg: github.com/ozontech/file.d/plugin/input/file
BenchmarkLightJsonReadPar-8            1        1841799875 ns/op         787.94 MB/s     1192072 B/op       2224 allocs/op
--- BENCH: BenchmarkLightJsonReadPar-8
    file_test.go:1150: using low memory pool
    file_test.go:1187: Memory used: 90MiB