Adding sync.Pool to Compress Middleware

[pafuent]

Adding a sync.Pool for the *gzip.Writer reduces the allocations of the Compress middleware in 50% and gives an increase on execution speed of a 85%
This fix #1643

Here are the Benchmarks

Current version

PASS ok github.com/labstack/echo/v4 0.004s goos: linux goarch: amd64 pkg: github.com/labstack/echo/v4/middleware BenchmarkGzip-2 7473 151518 ns/op 815268 B/op 33 allocs/op PASS ok github.com/labstack/echo/v4/middleware 1.901s 

Using a sync.Pool

PASS ok github.com/labstack/echo/v4 0.005s goos: linux goarch: amd64 pkg: github.com/labstack/echo/v4/middleware BenchmarkGzip-2 47617 23736 ns/op 1430 B/op 16 allocs/op PASS ok github.com/labstack/echo/v4/middleware 1.395s 

[codecov]

Codecov Report

Merging #1672 (ac54e13) into master (ceffc10) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## master #1672 +/- ## ========================================== + Coverage 84.06% 84.35% +0.29%  ========================================== Files 28 28 Lines 1901 1911 +10 ========================================== + Hits 1598 1612 +14  + Misses 191 189 -2  + Partials 112 110 -2 

Impacted Files	Coverage Δ
middleware/compress.go	84.31% <100.00%> (+13.58%)	⬆️

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[imxyb]

Very correct, this is exactly what I think

[lammel]

Impressive! Very clean patch, thanks @pafuent