Allow pipelining of method calls

[cdickmann]

I propose the following:

• Start using the net-http-pipeline gem See:
  • http://redmine.ruby-lang.org/issues/5461
  • https://github.com/drbrain/net-http-pipeline/blob/master/lib/net/http/pipeline.rb
  • http://www.igvita.com/2011/10/04/optimizing-http-keep-alive-and-pipelining/
• Offer Connection#_call version that is split into two parts
• Connection#_callPipelined would build the request and add it to the pipeline. It would internally maintain a FIFO queue of in-flight requests. The caller would be returned with a dummy object that offers DummyResult#result which will block and return the result (when received)
• A call to DummyResult#result or a call to Connection#_call would both trigger processing of the responses queued up in the TCP connection
• Offer new methods on ManagedEntity called _Pipelined that take the same arguments as the normal methods but instead of calling Connection#_call they call Connection#_callPipelined and return the DummyResult object.