Item needs stringing.
Periodically clean up expired items.

cbugg: close bug-398 bug-339
Memory-only bucket metadata survives server restart.

cbugg: closes bug-402

Only the important metadata state (e.g., VBActive) and # of vbuckets
survives the restart.  The in-memory key-value items & changes-stream,
of course, are lost during a restart.
TODO comment tweaks.