thread state coordinator: added a thread resume request and related tests.

The thread resume block is executed in the normal flow of thread
state queued event processing.  The tests verify that it is executed
when we track the thread to be stopped and skipped when we track
it to already be running.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218638 91177308-0d34-0410-b5e6-96231b3b80d8