MB-8496: Don't require a value for non-resident meta-data

This change removes the requirement that a non-resident item
needs to have a blob that contains the valur length. This
change introduces a new flag that signals if an item has been
deleted. This change will also reduce meta data by 16 bytes for
each non-resident meta item.

Change-Id: I0d8cbcfba6415c5ff1d61610c783ae517b48848c
Reviewed-on: http://review.couchbase.org/27034
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>