Change 'lcb_wait' semantics

Currently we have 'wait' and 'breakout' which is optimized and termed for a synchronous model.

We should rename these functions (though we can still maintain aliases) to something like:

lcb_operations_flush:

Which indicates that the user has finished sending requests, and that packets should be sent

And:
lcb_operations_done_callback

Which will be called when the operations have been completed.

Effectively this is the same as lcb_wait and the stop_event_loop callback, but the usage and purpose of those two existing functions aren't clear.