Patch for hot table optimization

MySQL 5.5 grabs the MDL_SHARED_NO_READ_WRITE for optimize table. TokuDB's optimize table allows concurrent reads and writes. This patch allows the storage engine to tell MySQL to downgrade the MDL to MDL_SHARED_WRITE before optimize table really does its work. See https://github.com/Tokutek/tokudb-engine/wiki/Optimize-Table for details.

In MySQL 5.6 and MariaDB 10.0, optimize table can be transformed into a alter table reorganize table and analyze table. Since alter table reorganize table can manipulate the MDL, this patch is not necessary.