Temporarily roll back the parts of r211241 where I fixed a
mistake in the lock acquistion in HistoryUnwind and HistoryThread.
We've got a deadlock with one use case of HistoryUnwind; I
need to figure out what lock ordering is causing this and fix
it for real.  
<rdar://problem/17411904> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211541 91177308-0d34-0410-b5e6-96231b3b80d8