PYTHON-526 Remove 'compile_re' option.

PyMongo now never attempts to compile BSON regular expressions as Python native
regular expressions.
Rename maybe_return_socket to return_socket.

The name "maybe_return_socket" made more sense when a socket might *not* be
returned because it was a request socket, or because the pool might be
overfull. Now requests are gone, and the pool can't be overfull--"force" is
gone and max_pool_size is strictly enforced.