Add the 'connect_timeout' and 'request_max_timeout' configuration
fields, to control respectively the initial connection timeout, and
the maximum time for each individual request. This allows fine-tuning
of the expected performance of specific backends, for example to let
optional backends fail fast.

Trace requests on client and server HTTP traffic, using a system-wide
configuration file to control reporting.

Implement a simpler API for the Backend interface, removing most old
public methods and replacing them with a single Call() method, making
the package look a bit more like an actual rpc package (so, hopefully,
easier to replace in the future).