Refactor stream reading

[j-vizcaino]

This PR refactors how data is read and uncompressed.

It adds a doubleBuffer helper struct to do the magic C, double buffer dance and makes use of this in the Read function.

Here are the benchmarks comparing the current master with this branch:

master

$ go test -bench='^BenchmarkStreamUncompress.*' -benchtime 15s -count 2
goos: darwin
goarch: amd64
pkg: github.com/DataDog/golz4
BenchmarkStreamUncompress-4   	 5527802	      4236 ns/op	7735.24 MB/s	     123 B/op	       2 allocs/op
BenchmarkStreamUncompress-4   	 3750264	      6997 ns/op	4682.91 MB/s	     142 B/op	       2 allocs/op
PASS
ok  	github.com/DataDog/golz4	65.629s

New version

$ go test -bench='^BenchmarkStreamUncompress.*' -benchtime 15s -count 2
goos: darwin
goarch: amd64
pkg: github.com/DataDog/golz4
BenchmarkStreamUncompress-4   	 5643277	      4133 ns/op	7927.65 MB/s	     126 B/op	       3 allocs/op
BenchmarkStreamUncompress-4   	 3755970	      6837 ns/op	4792.58 MB/s	     140 B/op	       3 allocs/op
PASS
ok  	github.com/DataDog/golz4	65.232s

Performance is very similar, sometimes a bit better. There's still an extra alloc that slipped through the cracks, probably related to the addition of doubleBuffer.

FWIW, I also tested moving the temporary readBuffer as a struct member but this turned out to be worse, performance wise

[j-vizcaino]

Wrong target repo.