Lock out Process::RunThreadPlan so only one can be in flight at a time.

What with all sorts of folks (TSAN, ASAN, queue detection, etc...) trying to
gather info by calling functions down in the lower layers of lldb, we've started
to see people running expressions simultaneously.  The expression evaluation part
is okay, but only one RunThreadPlan can be active at a time.  I added a lock to
enforce that.

<rdar://problem/26431072>

llvm-svn: 270593