Log: Fix race in accessing the stream variable

Summary:
The code was attempting to copy the shared pointer member in order to
guarantee atomicity, but this is not enough. Instead, protect the
pointer with a proper read-write mutex.

This bug was present here for a long time, but my recent refactors must
have altered the timings slightly, such that now this fails fairly often
when running the tests: the test runner runs the "log disable" command
just as the thread monitoring the lldb-server child is about to report
that the server has exited.

I add a test case for this. It's not possible to reproduce the race
deterministically in normal circumstances, but I have verified that
before the fix, the test failed when run under tsan, and was running
fine afterwards.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D30168

llvm-svn: 295712