Use forward_iterator_tag for histogram

Looks like we're missing some methods in order to be
a random iterator (getting tons of errors with MSVC).
Since we're not using them in both directions anyway
the easiest solution was just to move it to a forward
iterator (errors from template code generates a LOT
of unreadable errors ;-) )

Change-Id: I9a8df00b6b18b6096336cffcb31d09a88cdbf044
Reviewed-on: http://review.couchbase.org/31327
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>