TDB-114 (Change use of MySQL HASH to unordered_map) introduces memory leak

Previously, in drop_share, removing a share from the HASH called the share->destroy method which cleaned up. When converted to std::unordered_map, the share must be explicitly destroyed on removal.

This looks limited to a DROP TABLE or ALTER TABLE ... RENAME as they are the only known pathc through TOKUDB_SHARE :: drop_share