refactor: Use sync.Pool to reuse the []byte with specific length

[Zheaoli]

在 #747 后，可以对 RDB Parse 做个基准 Benchmark

原本直觉上 ParseRDB 的频繁小内存分配会成为内存瓶颈，但是实际上池化的代价更大一些

# before
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
BenchmarkParseRDB-16                  20         150637274 ns/op        213802609 B/op   1900710 allocs/op
BenchmarkParseRDB-16                  20         155507008 ns/op        213802064 B/op   1900714 allocs/op
BenchmarkParseRDB-16                  20         151941175 ns/op        213801739 B/op   1900715 allocs/op
BenchmarkParseRDB-16                  20         151335552 ns/op        213801515 B/op   1900718 allocs/op
BenchmarkParseRDB-16                  20         148564198 ns/op        213801514 B/op   1900716 allocs/op


# after

cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
BenchmarkParseRDB-16                  40         152801292 ns/op        212345090 B/op   1481219 allocs/op
BenchmarkParseRDB-16                  40         159080792 ns/op        212362787 B/op   1481263 allocs/op
BenchmarkParseRDB-16                  40         158858734 ns/op        212349859 B/op   1481234 allocs/op
BenchmarkParseRDB-16                  40         157454181 ns/op        212330968 B/op   1481179 allocs/op
BenchmarkParseRDB-16                  40         156932052 ns/op        212318951 B/op   1481155 allocs/op