sessiond: rename locked_ptr to locked_ref

sessiond: rename locked_ptr to locked_ref

The locked_ref semantics garantee that an instance of an is locked and that it
exists (it can't be null). It is currently implemented with a unique_ptr, but
that will most likely change in the future to enforce the non-null requirement.

Change-Id: I6442840f944ded1ddc46cad3d23e8e0fe8fef586
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>