[Reviewer Rob] Issue144 - improvements to HTTP connection failure loggin

Rob

As discussed, these are my fixes to improve the logging and retry behaviour in httpconnection.cpp.  I don't think they completely fix issue 144, but they definitely improve the behaviour and give us more of a chance of getting to the bottom of the underlying issue.

Mike

I got a bit carried away while doing updates and sorted out most of the things I didn't like about this class - I moved PoolEntry to be a subclass, made the callback's from CURL static member functions of the class rather than bare functions, and move things around better the header and the cpp file.  The only thing I didn't fix is the "#pragma once" and the fact that this class should be inherited by it's users (which would get rid of the rubbish like having to pass a SAS event base number in.

I've also fixed up the UTs, so we're back to 100% code coverage.