Implement custom healthcheck middleware

[fvennetier]

This extension of swift's healthcheck middleware counts the number of requests currently being processed. The counter can be retrieved from a JSON object generated when calling the /_status route (can be changed by configuration).

oioswift processes can be registered in oio-sds' conscience with watchers configured like this:

host: 10.0.3.1
port: 5000
type: oioswift
service_id: oioswift-1
location: rack.127-0-0-1.novol
checks:
    - {type: http, uri: "/healthcheck"}
slots:
    - oioswift
stats:
    - {type: system}
    - {type: http, path: "/_status", parser: json}

[codecov-io]

Codecov Report

Merging #202 into master will decrease coverage by 2.09%. The diff coverage is 31.09%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
- Coverage   76.04%   73.96%   -2.08%     
==========================================
  Files          36       37       +1     
  Lines        3793     3843      +50     
==========================================
- Hits         2884     2842      -42     
- Misses        909     1001      +92

Impacted Files	Coverage Δ
oioswift/common/middleware/healthcheck.py	0% <0%> (ø)
oioswift/__init__.py	100% <100%> (ø)	:arrow_up:
oioswift/server.py	87.1% <100%> (+0.66%)	:arrow_up:
bin/oioswift-proxy-server	55.89% <55.89%> (ø)
oioswift/common/middleware/tempauth.py	0% <0%> (-39.13%)	:arrow_down:
oioswift/common/middleware/container_hierarchy.py	66.32% <0%> (-6.17%)	:arrow_down:

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