Split lock() into "just lock" and "update entry reference time" interfaces.

Split lock() into "just lock" and "update entry reference time" interfaces.
Improved entry lock/unlock debugging.

Several modern store entry users need to lock the entry for storage rather
then to update entry's reference time. There is an old XXX describing the
corresponding API bug that merges the two distinct operations. This change
does not modify existing code, but allows new code to use new/fixed API
instead of changing an essentially private entry lock counter.