[Reviewer Matt] Select a transport from connection pool for routing calls to sprout

Matt

Can you review this change - this is just reintroducing the call to assign a transport from the upstream connection pool when Bono is routing upstream to Sprout.  I've tested it by kicking over one of a three-node cluster of Sprouts and checking that only a small numbers of calls failed (without the fix it is utter carnage).

It wasn't good that this regression got past the unit tests, so I have taken an action to look at beefing up some of the checking.

Mike