Collection.get_and_lock should not get and lock in 2 commands

Seems it currently was implemented to make 2 calls, rather than one. But most if not all we need for one call seems to exist in the bindings. Lets add a get_and_touch (note there is a PYCBC_CMD_GAT which fits the bill, we probably just need to add a function that maps to it, and the be sure it actually works).

The current work that touches on get_and_lock (along with others) – PYCBC-818 – should be done first. We could do this right after, or depending on other priorities it could be done later.