Switch to notify, not sync_notify.

Despite the commit message on d19d927da6, I can't see how sync_notify
could possibly be faster, and I'm seeing performance issues when logging.
Fix tests matching previous changeset.

Now that we are logging asynchronously, we need to add flushes in critical
places in the tests.
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/lager.erl
	src/lager_file_backend.erl
Reapply the changes to switch from sync_notify to notify, following
the merge of basho/master.
Remove quotes from around strings.
Revert "Remove quotes from around strings."

I've decided that the bug lies in my log.erl, not lager.

This reverts commit 90c7169553bb27b4487145482eda3850d0f61cf2.
patched origin with sync_notify
Merge commit '467204dbbf0f50d98909f60a67db9e47c4a79be1'