Converted MemoryOnly into level instead of bool.
Feature to persist nothing.

Use command-line flag of -default-memory-only=2 to have a higher level
of non-persistence (persist nothing / don't touch disk).

A flag of -default-memory-only=1 means persist only bucket metadata,
but do not persist item operations; which means the bucket will
survive a server restart but will have zero items.

cbugg: close bug-407