Fix a race condition in lldb-mi.

lldb-mi has 3 threads.
1. One that waits for user intput.
2. Another waits for 1st thread to get input command.
3. Waits for events from lldb.

2 & 3 needs to be synchronized so that they don't end up
doing things at same time. For example, while "break insert" is
processing, we can get a breakpoint event. Depending on where we
are in "break-insert", it can have different behavior. In some
cases, it can cause breakpoint to be deleted too. I have added a 
mutex so that command processing and event processing are done 
exclusively and they are not running at the same time.

In longer term, I think thread 2 & 3 should be merged to be only 
one thread which can wait on command or events.

Reviewed in http://reviews.llvm.org/D7371.

llvm-svn: 228128