Packet pool optimisations

[k-danil]

Good day!

This commit introduce two optimisations for Packet pool:

• Replace parseData with isPSIComplete function. This function checks for advertised section length and compare it with length of actual data in accumulator.
• Allocate slices with capacity of previous one. For PES packets this will significantly reduce reallocation count.

Benchmarks:
Now:

BenchmarkDemuxer_NextData
BenchmarkDemuxer_NextData-8   	  102381	     11409 ns/op	   15247 B/op	     300 allocs/op
PASS

Patch:

BenchmarkDemuxer_NextData
BenchmarkDemuxer_NextData-8   	  263058	      4471 ns/op	    6666 B/op	     125 allocs/op
PASS

Please check.

[asticode]

Cheers