Handle buffer pool for oxy

[juliens]

What does this PR do?

Add a buffer pool to handle the copy of body in oxy.
vulcand/oxy#138

Motivation

Better performance
Fixes #3422

More

Some benchmark:
I use emilevauge/whoami binary, and traefik binary (without docker)

$ ./whoamI -port 8090

traefik.toml

maxIdleConnsPerHost = 100000
defaultEntryPoints = ["http"]

loglevel="ERROR"

[entryPoints]
  [entryPoints.http]
  address = ":8081"

[file]
  [frontends.frontend-f]
    backend = "b" 
  [frontends.frontend-f.routes.test_1]
  rule = "Host:localhost"
  [backends]
    [backends.b]
        [backends.b.servers.website]
            url = "http://127.0.0.1:8090"

Træfik v1.4

$ wrk -t20 -c1000 -d60s -H "Host: localhost" --latency  http://127.0.0.1:8081/bench
Running 1m test @ http://127.0.0.1:8081/bench
  20 threads and 1000 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    30.45ms   21.50ms 467.58ms   61.19%
    Req/Sec     1.69k   241.43     7.55k    88.77%
  Latency Distribution
     50%   29.68ms
     75%   44.56ms
     90%   56.40ms
     99%   84.52ms
  2013008 requests in 1.00m, 195.81MB read
Requests/sec:  33499.60
Transfer/sec:      3.26MB

PR version

$ wrk -t20 -c1000 -d60s -H "Host: localhost" --latency  http://127.0.0.1:8081/bench
Running 1m test @ http://127.0.0.1:8081/bench
  20 threads and 1000 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    27.10ms   15.42ms 208.96ms   70.76%
    Req/Sec     1.87k   209.22    11.31k    82.94%
  Latency Distribution
     50%   28.99ms
     75%   36.52ms
     90%   44.77ms
     99%   63.80ms
  2237140 requests in 1.00m, 217.62MB read
Requests/sec:  37222.81
Transfer/sec:      3.62MB

[ldez]

LGTM

[dtomcej]

LGTM

[mmatur]

LGTM