Fix remaining threading issues in Log.h

Summary:
This fixes two threading issues in the logging code. The access to the
mask and options flags had data races when we were trying to
enable/disable logging while another thread was writing to the log.
Since we can log from almost any context, and we want it to be fast, so
I avoided locking primitives and used atomic variables instead. I have
also removed the (unused) setters for the mask and flags to make sure
that the only way to set them is through the enable/disable channel
functions.

I also add tests, which when run under tsan, verify that the use cases
like "doing an LLDB_LOGV while another thread disables logging" are
data-race-free.

Reviewers: zturner, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D30702

llvm-svn: 297368