Change the loopmgr to be singleton

Change the loopmgr to be singleton

All the applications built on top of the loop manager were required to
create just a single instance of the loop manager.  Refactor the loop
manager to not expose this instance to the callers and keep the loop
manager object internal to the isc_loop compilation unit.

This significantly simplifies a number of data structures and calls to
the isc_loop API.