Couchbase++ with Transactions 2.1

This Epic represents the first milestone for the CB++ project that will be in a good enough state to refactor the wrappers and implement support for Transactions 2.1 as well.

This does not include a publicly released CB++ SDK (just yet), nor does in include making a new version of LCB based on the new CB++ (just yet). Those will be future goals, but this first phase is really all about providing an internal version that has what the wrappers need to implement transactions 2.1 properly.