UCT/IB: Fix teardown path on memory handler callback

[tvegas1]

What

Fix error handling path on multi-threaded registration.

Why ?

Currently we see segfault in the thread because we try to teardown the first failing registration. More generally, a thread can fail to start or fail to properly register all its tasked mrs.

How ?

• Make sure we cannot dereg twice.
• Make sure we only teardown mrs entries corresponding to started threads.
• Make sure started threads return with their mrs all initialized.