Second pass on limiting the public interface

Someone using transactions only really needs transactions, transaction_config, and attempt_context, and perhaps the public exception classes. So ideally, that is all transactions.hxx would include, and all that would appear in include/couchbase/transactions.

For example although transactions.run returns a transaction_result, it isn't truly a user-facing thing (only for tests). Lets forward declare it, move it into the src (along with the rest.

Also – we need to adjust the Doxyfile to only generate docs on public stuff (for now). Unless we go open-source, no need to document internals.