Fixed deadlocks that could occur when using python for breakpoints, operating system plugins, and other async python usage.

<rdar://problem/16054348>
<rdar://problem/16040833>



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201372 91177308-0d34-0410-b5e6-96231b3b80d8