use lockless sequence in itimer signal handler

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on November 01, 2011 17:09:05_

xref Dr. Memory issue #626

currently the alarm handler tries to grab a lock to avoid clobbering
a change to the time-left counter on an interval change by another thread.

also, it currently continues even when it can't get the lock after 2 tries:
which perhaps should be re-thought, but that's orthogonal to the main point
of this issue, which is to come up with a lockless protocol for updating
the multiplexing timer state.

idea: count up and mod to avoid having to reset?
need to figure out how to handle intervals w/o a simple common factor
though.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=597_