This is needed because when e.g. wcfs is just started the coverage of
ΔFtail is (head,head] i.e. empty, and if user wants to setup a watch
with at=head, it becomes watch with at=tail. Then that at is used in a
query and if point-queries with at=tail are disallowed it panics with
"at out of bounds".

This fixes crashes in test_wcfs_watch_setup (see 339f1884 "wcfs: tests:
Always start tDB with ZBigFile pre-created before WCFS startup") and in
test_wcfs_crash_old_data (see 97ce5105 "wcfs: tests: Add test do
demonstrate "at out of bounds" crash on readPinWatchers ->
ΔFtail.BlkRevAt")

For the reference zodb.ΔTail already allows point queries with at=tail:

https://lab.nexedi.com/kirr/neo/blob/1193c44e/go/zodb/δtail.go#L202-206
https://lab.nexedi.com/kirr/neo/blob/1193c44e/go/zodb/δtail.go#L225-228