Create `Locker.encache` and use it in `create`.

Place `encache` inside `Locker` instead of having an additional function
outside `Locker` that also manipulates the queue. No reason for it not
to be inside `Locker` and for it to not also hold a lock on the encached
page's lock queue.

Now using `Locker.encache` in `create`.

Closes #288.
Closes #287.