The implementation of handlerton::drop_database in InnoDB is
unnecessarily complex. The minimal implementation should check
that no conflicting locks or references exist on the tables,
delete all table metadata in a single transaction, and finally
delete the tablespaces.

Note: DROP DATABASE will delete each individual table that the
SQL layer knows about, one table per transaction.
The handlerton::drop_database is basically a final cleanup step
for removing any garbage that could have been left behind
in InnoDB due to some bug, or not having atomic DDL in the past.

hash_node_t: Remove. Use the proper data type name in pointers.

dict_drop_index_tree(): Do not take the table as a parameter.
Instead, return the tablespace ID if the tablespace should be dropped
(we are dropping a clustered index tree).

fil_delete_tablespace(), fil_system_t::detach(): Return a single
detached file handle. Multi-file tablespaces cannot be deleted
via this interface.

ha_innobase::delete_table(): Remove a work-around for non-atomic DDL
and do not try to drop tables with similar-looking name.

innodb_drop_database(): Complete rewrite.

innobase_drop_database(), dict_get_first_table_name_in_db(),
row_drop_database_for_mysql(), drop_all_foreign_keys_in_db(): Remove.

row_purge_remove_clust_if_poss_low(), row_undo_ins_remove_clust_rec():
If the tablespace is to be deleted, try to evict the table definition
from the cache. Failing that, set dict_table_t::space to nullptr.

lock_release_on_rollback(): On the rollback of CREATE TABLE, release all
locks that the transaction had on the table, to avoid heap-use-after-free.