elasticsearch: Made RequestsFuturesHttpConnection pass bulk index test

* Changed return type of perform_request.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
elasticsearch: Partial implementation of Transport class

* Created a Transport (sub)class to manage our async connections,
  since the standard Transport relies on synchronous returns.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>