Add sync.Pool to gzip middleware

[tylerstillwater]

Using a sync.Pool and the gzip.Writer.Reset method, we are able to reduce allocations, memory usage, cpu time and gc time. Benchmark below:

benchmark         old ns/op     new ns/op     delta
BenchmarkGzip     321829        94806         -70.54%

benchmark         old allocs     new allocs     delta
BenchmarkGzip     79             58             -26.58%

benchmark         old bytes     new bytes     delta
BenchmarkGzip     1459072       8065          -99.45%

[vishr]

Nice!