BUG/MEDIUM: stick-tables: fix the task's next expiration date

BUG/MEDIUM: stick-tables: fix the task's next expiration date

While changing the stick-table indexing that led to commit 1a088da7c
("MAJOR: stktable: split the keys across multiple shards to reduce
contention"), I met a problem with the task's expiration date being
incorrectly updated, I fixed it and apparently I committed the wrong
version :-/

The effect is that the task's date is only correctly reset if the
table is empty, otherwise the task wakes up again and is queued at
the previous date, eating 100% CPU. The tick_isfirst() must not be
used when storing the last result.

No backport is needed as this was only merged in 3.0-dev7.