Pool Key lists and buffers in Pebble store

[masih]

Pool buffers, keys, keyer and key list in order to reduce CPU and memory footprint in Pebble store implementation.

Increase merger efficiency by avoiding recursion when flattening marshalled value keys. Instead, use mod to skip to where the serialized value keys should begin. This works because varint length is always added to the begining of value keys and their length is fixed in pebble store to 20 + 1.

During merge any value keys with unexpected key prefix are skipped. This automatically removes such keys from the store as they will be excluded from the merge.

[masih]

Workload 0 benchmark for comparison:

$ go test ./bench -bench='(Pebble|Storethehash)(Put|Get)_W(0)'  -count=5 -timeout=300m 

BenchmarkStore_PebblePut_W0-8                  1        1988610008 ns/op          17.15 MB/s      48424081 size 201391560 B/op   4329444 allocs/op
--- BENCH: BenchmarkStore_PebblePut_W0-8
    store_bench_workloads_test.go:33: Workload 1 generated 1000 values with total size of 32.522 MiB.
BenchmarkStore_PebblePut_W0-8                  1        1686782294 ns/op          20.22 MB/s      48417771 size 195709632 B/op   4192982 allocs/op
BenchmarkStore_PebblePut_W0-8                  1        1643019567 ns/op          20.76 MB/s      48424081 size 195539184 B/op   4192796 allocs/op
BenchmarkStore_PebblePut_W0-8                  1        1693129651 ns/op          20.14 MB/s      48417771 size 193930168 B/op   4156038 allocs/op
BenchmarkStore_PebblePut_W0-8                  1        1706912678 ns/op          19.98 MB/s      48424081 size 194735544 B/op   4177132 allocs/op
BenchmarkStore_PebbleGet_W0-8                  1        6024332750 ns/op           5.66 MB/s      45466996 size 959914736 B/op  21814745 allocs/op
BenchmarkStore_PebbleGet_W0-8                  1        6013348175 ns/op           5.67 MB/s      45466996 size 959271576 B/op  21798756 allocs/op
BenchmarkStore_PebbleGet_W0-8                  1        6069595870 ns/op           5.62 MB/s      45466996 size 960972840 B/op  21835899 allocs/op
BenchmarkStore_PebbleGet_W0-8                  1        5990691385 ns/op           5.69 MB/s      45466996 size 960192128 B/op  21815468 allocs/op
BenchmarkStore_PebbleGet_W0-8                  1        6145057088 ns/op           5.55 MB/s      45466996 size 960327352 B/op  21821678 allocs/op
BenchmarkStore_StorethehashPut_W0-8            1        3722073469 ns/op           9.16 MB/s      87396304 size 973280640 B/op  11670663 allocs/op
BenchmarkStore_StorethehashPut_W0-8            1        3630226099 ns/op           9.39 MB/s      87392708 size 986805832 B/op  11704640 allocs/op
BenchmarkStore_StorethehashPut_W0-8            1        3680018004 ns/op           9.27 MB/s      87396126 size 989693960 B/op  11747880 allocs/op
BenchmarkStore_StorethehashPut_W0-8            1        3343566343 ns/op          10.20 MB/s      87378262 size 968908464 B/op  11908723 allocs/op
BenchmarkStore_StorethehashPut_W0-8            1        3314806176 ns/op          10.29 MB/s      87376078 size 970933888 B/op  11963848 allocs/op
BenchmarkStore_StorethehashGet_W0-8            1        9823852154 ns/op           3.47 MB/s      87374876 size 1778294872 B/op 32526981 allocs/op
--------------------------------------------------------
Store_PebblePut_W0-8           1.74s ±14%
Store_PebbleGet_W0-8           6.05s ± 2%
Store_StorethehashPut_W0-8     3.54s ± 6%
Store_StorethehashGet_W0-8     9.71s ± 4%

name                        speed
Store_PebblePut_W0-8        19.6MB/s ±13%
Store_PebbleGet_W0-8        5.64MB/s ± 2%
Store_StorethehashPut_W0-8  9.66MB/s ± 6%
Store_StorethehashGet_W0-8  3.51MB/s ± 4%

name                        size
Store_PebblePut_W0-8           48.4M ± 0%
Store_PebbleGet_W0-8           45.5M ± 0%
Store_StorethehashPut_W0-8     87.4M ± 0%
Store_StorethehashGet_W0-8     87.4M ± 0%

name                        alloc/op
Store_PebblePut_W0-8           196MB ± 3%
Store_PebbleGet_W0-8           960MB ± 0%
Store_StorethehashPut_W0-8     978MB ± 1%
Store_StorethehashGet_W0-8    1.75GB ± 3%

name                        allocs/op
Store_PebblePut_W0-8           4.21M ± 3%
Store_PebbleGet_W0-8           21.8M ± 0%
Store_StorethehashPut_W0-8     11.8M ± 1%
Store_StorethehashGet_W0-8     32.0M ± 3%

[gammazero]

LGTM