Cleanup adapter connections

[technoweenie]

This adds #build_connection and #connection to the adapters, as suggested in #1006 (comment).

• TODO
  • excon
  • httpclient
  • net_http_persistent
  • net_http
  • patron
• SKIP
  • EM HTTP - Is external pooling needed with the EM reactor?
  • EM Synchrony - EM reactor
  • rack - No actual HTTP connection
  • test - No actual HTTP connection
  • typhoeus - Maintained externally. Can file patch later!

[technoweenie]

Fixed this faraday-live failure in f8615d5:

$ FARADAY_GEM_REF=ba7b82d9a2211c4fdd840189e246dbd7a6e4c229 docker-compose build tests
$ docker-compose run tests

  1) net_http_persistent with unverified HTTPS server succeeds with verification disabled
     Failure/Error: res = conn.get('unverified_with_verification')

     Faraday::SSLError:
       SSL_connect returned=1 errno=0 state=error: certificate verify failed (unable to get local issuer certificate)

[iMacTia]

Standardising the different adapters can only benefit us in the future, so big 👍 from me

[olleolleolle]

I have proof-read all this, and it looks clear.

The new supporting methods help a lot!