Removed sleep/purge goroutines.

cbugg: closes bug-372

The purge-old-files feature now seems broken, where after compacting a
bucketstorefile, the finalizer hook does not seem to work or perhaps
be invoked.  Hence, skipping this test...

   compact_test.go / SKIP_TestCompactionPurgeTimeout()

Even so, checking this in so that Dustin isn't blocked.
Updated the features docs.