PT-1163 Fixed memory leak caused by creating eternal goroutines for every new transport

[vgarvardt]

What does this PR do?

This PR solves the problem with memory leaking on each reouter reload. The problem was in the transport - each definition created own transport, and each launched infinite go-routine. The solution is the following: do not create transport for each route, reuse one from the registry instead (registry key is being built from transport parameters), and do not launch go-routine, let transport retire "keep-alive" connections using internal transport functionality.

In addition to this there are couple minor changes I did during issue investigation:

• added pprof http handlers to private port, can be enabled only with command arguments
• replaced unmaintained gopkg.in/mgo.v2 with community-maintained fork github.com/globalsign/mgo - profiling shown that mongo stuff (like BSON-decoding) consumes quite a lot of resources for ~100 proxies loaded every second from Mongo

One known issue is still not fixed - the way I discovered this memory leak issue is that reflect.DeepEqual() does not work properly when comparing current loaded from Mongo data in our environment. I'm in the process of investigation, but let it be in the next PR.

[codecov[bot]]

Codecov Report

Merging #328 into master will decrease coverage by 0.21%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
- Coverage   41.47%   41.25%   -0.22%     
==========================================
  Files          74       74              
  Lines        2486     2499      +13     
==========================================
  Hits         1031     1031              
- Misses       1390     1403      +13     
  Partials       65       65

Impacted Files	Coverage Δ
pkg/plugin/oauth2/mongodb_repository.go	0% <ø> (ø)	:arrow_up:
pkg/plugin/oauth2/setup.go	7.86% <ø> (ø)	:arrow_up:
pkg/router/listen_path_matcher.go	100% <ø> (ø)	:arrow_up:
pkg/config/specification.go	61.11% <ø> (ø)	:arrow_up:
pkg/api/mongodb_repository.go	0% <0%> (ø)	:arrow_up:
pkg/web/options.go	0% <0%> (ø)	:arrow_up:
pkg/proxy/register_options.go	0% <0%> (ø)	:arrow_up:
pkg/proxy/register.go	0% <0%> (ø)	:arrow_up:
pkg/web/provider.go	0% <0%> (ø)	:arrow_up:
pkg/plugin/basic/mongodb_repository.go	4.34% <0%> (ø)	:arrow_up:

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