The problem is that during 'ALTER TABLE .. TRUNCATE PARTITION ...',
server will keep a handler open, and ask SE to do the real truncation,
however, it's quite difficult for SE to handle errors if SE is
truncating the partition in the DROP+CREATE way. If there is error after
SE dropped one or more partitions, there is no easy way to re-open the
whole table.

So the solution is that close all handlers before diving into SE to
truncate partitions, and let SE to open and manage each partition.

This patch also cleans up some related code, and another bug gets fixed
too is that truncating some partitions will not affect current
AUTO_INCREMENT value, even if the partition containing the max value
is truncated. So only truncate the whole table will reset the
AUTO_INCREMENT value to 0, otherwise, it should be kept as is.

RB: 19371
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>