Add ΔTail - utility class to keep track of history tail of revisional
changes. This will be needed for both DB (to handle invalidations) and
for raw Cache(*). It also might be useful in places outside of zodb/go -
for example WCFS from Wendelin.core uses ΔTail to keep track of which
ZODB revision changed which block of file(+).

Please see ΔTail comments for details.

(*) to check a bit ahead on load to see whether loaded serial is
actually the latest one for ≤ Cache.head, and, if yes, mark
corresponding cache entry as having .head="current" so that the entry
coverage could be extended when Cache.head is extended to cover more
database state instead of loading the same data from the database again.

(+) that's why we have δtail.go.cat-generic, so that third-party
packages could adapt ΔTail to needed types.