Renamed the periodic execuctors for stats.

every* is too generic, and not necessarily accurate.
Test shouldContinueDoingStats
Fix write-only checking in FileLike.
Rewrote item serializer to be simpler and faster.

There were some error conditions that were pretty well impossible and
not covered.  Simplifying it made those errors go away, increased
coverage and should speed it up quite a bit.
Turn off SET_VBMETA processing.

The initial release of cbgb for DBaaS should only have active
vbuckets, where clients shouldn't be able to remotely change vb
state/meta info.
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.