Move the thread logic around to fit better into LLDB's process model.

Previously we were directly updating the thread list and stopping
and restarting the process every time threads were created.  With
this patch, we queue up thread launches and thread exits, resolve
these all internally, and only update the threads when we get an
UpdateThreadList call.  We now only update the private state on
an actual stop (i.e. breakpoint).

llvm-svn: 222178