MAINT-5271: Converted internal pointers to internal operation to managed shared pointers. Removed direct cast and dereference of handles.

Well, achieved that by doing work in bulk when needed.  But
turned into some additional things.  Change timebase from
mS to uS as, well, things are headed that way.  Implement
an HttpReplyQueue::fetchAll method (advertised one, hadn't
implemented it).

SH-3184/SH-3221  More work on cleanup with better unit test work and more aggressive shutdown of a thread.
Some additional work let me enable a memory check for the clean shutdown case and
generally do a better job on other interfaces.  Request queue waiters now awake
on shutdown and don't sleep once the queue is turned off.  Much better semantically
for how this will be used.

Identified and reacted to the priority inversion problem we
have in texturefetch.  Includes the introduction of a priority_queue
for the requests that are ready.  Start some parameterization in
anticipation of having policy_class everywhere.  Removed _assert.h
which isn't really needed in indra codebase.  Implemented async
setPriority request (which I hope I can get rid of eventually along
with all priorities in this library).  Converted to using unsigned
int for priority rather than float.  Implemented POST and did
groundwork for PUT.

The unit/integration tests don't work yet as I'm still battling cmake/autobuild
as usual but first milestone passed.