Do not merge tablets if that would drop the tablet_count
below the minimum provided by hints.
Split tablets if the current tablet_count is less than
the minimum tablet count calculated using the table's tablet options.
TODO: override min_tablet_count if the tablet count per shard
is greater than the maximum allowed. In this case
the tables tablet counts should be scaled down proportionally.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>