Speed up LogTest.

LogTest was quite slow due to heavy use of Thread.sleep(). I sped things
up by replacing the sleeps with countdown latches.
Add test for unregistering a log reader.

Added a quick unit test for the Log.unregisterReader(...) method.
Merge pull request #633 from solongordon/add-unregister-reader-test

Add test for unregistering a log reader.