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