Reduce allocs by pooling gzip.Writer objects

[shanson7]

Pretty simple change to pool for reuse. This came about because an instance of metric that was running had a pprof that looked like:

(pprof) top5
Showing nodes accounting for 21093.71GB, 64.73% of 32586.37GB total
Dropped 1236 nodes (cum <= 162.93GB)
Showing top 5 nodes out of 131
      flat  flat%   sum%        cum   cum%
10250.14GB 31.46% 31.46% 10888.51GB 33.41%  github.com/grafana/metrictank/vendor/github.com/klauspost/compress/flate.NewWriter /home/jenkins/workspace/metrictank-build/gopath/src/github.com/grafana/metrictank/vendor/github.com/klauspost/compress/flate/deflate.go
 3774.92GB 11.58% 43.04%  5570.56GB 17.09%  github.com/grafana/metrictank/vendor/github.com/eapache/go-xerial-snappy.Decode /home/jenkins/workspace/metrictank-build/gopath/src/github.com/grafana/metrictank/vendor/github.com/eapache/go-xerial-snappy/snappy.go
 3003.95GB  9.22% 52.26%  3003.95GB  9.22%  github.com/grafana/metrictank/vendor/github.com/Shopify/sarama.(*partitionConsumer).parseMessages /home/jenkins/workspace/metrictank-build/gopath/src/github.com/grafana/metrictank/vendor/github.com/Shopify/sarama/consumer.go
 2269.06GB  6.96% 59.22%  9842.42GB 30.20%  github.com/grafana/metrictank/vendor/github.com/Shopify/sarama.(*MessageBlock).decode /home/jenkins/workspace/metrictank-build/gopath/src/github.com/grafana/metrictank/vendor/github.com/Shopify/sarama/message_set.go
 1795.65GB  5.51% 64.73%  1795.65GB  5.51%  github.com/grafana/metrictank/vendor/github.com/golang/snappy.Decode /home/jenkins/workspace/metrictank-build/gopath/src/github.com/grafana/metrictank/vendor/github.com/golang/snappy/decode.go

Showing that flate.NewWriter was by far the largest source of allocations in our process. I tracked that to this component. I have already released this change to our production cluster and have seen a significant reduction and flate.NewWriter has completely dropped from the pprof output.

Benchcmp results:

benchmark           old ns/op     new ns/op     delta
BenchmarkGzip-8     207762        66689         -67.90%

benchmark           old allocs     new allocs     delta
BenchmarkGzip-8     50             34             -32.00%

benchmark           old bytes     new bytes     delta
BenchmarkGzip-8     1005656       4229          -99.58%

[replay]

Looks great, thanks