Refactor transport-http response delivery

Refactor transport-http response delivery

Using a ListenableFuture brings in lifecycle headache, as the reported
value is retained indefinitely -- forcing us to make copies of the
response.

Rework the APIs in terms of FutureCallback instead -- i.e. we report the
response while having a reference to it and the callback can retain it
if need be (in which case it takes ownership of it).

Also introduce ClientRequestDispatcher to eliminate a bit of duplicate
code.

Change-Id: I350467582b5f5bd67d5241d47a5d4cbaef19adaa
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>