MDEV-35055 ASAN errors in TABLE_SHARE::lock_share upon committing transaction after FLUSH on table with vector key

MHNSW_Trx cannot store a pointer to the TABLE_SHARE for the duration
of a transaction, because the share can be evicted from the cache any
time.

Use a pointer to the MDL_ticket instead, it is stored in the THD
and has a duration of MDL_TRANSACTION, so won't go away.

When we need a TABLE_SHARE - on commit - get a new one from tdc.
Normally, it'll be already in the cache, so it'd be fast.
We don't optimize for the edge case when TABLE_SHARE was evicted.