fix(storage/mongodb): use find + hint for delete

This patch replaces find_one in message delete with find().hint(). The
rationale behind this change is that I believe that find_one()
attempts to use all indices, so can incur some slowdown with
unnecessary scanning. find() + hint() avoids these additional scans.

Change-Id: I53193204a9a90958eadcfce8db14c4fc00a040de