Allow option to db.current_op() to include idle connections

db.current_op() currently only fetches the active operation. However the server can accept a "$all" flag which allows getting all the idle operations as well.

Currently the only way to do this is to query the virtual collection $cmd.sys.inprog directly with {$all:1} and _is_command=True, so we might as well just add this as an option to db.current_op itself.