Active transactions must not switch table or index definitions on the fly,
for several reasons, including the following:

 * copied indexes do not carry any history or locking information;
   that is, rollbacks, read views, and record locking would be broken

 * huge potential for race conditions, inconsistent reads and writes,
   loss of data, and corruption

Instead of trying to track down if the table was changed during a transaction,
acquire appropriate locks that protect the creation and dropping of indexes.

innodb-index.test: Test the locking of CREATE INDEX and DROP INDEX.  Test
that consistent reads work across dropped indexes.

lock_rec_insert_check_and_lock(): Relax the lock_table_has() assertion.
When inserting a record into an index, the table must be at least IX-locked.
However, when an index is being created, an IS-lock on the table is
sufficient.

row_merge_lock_table(): Add the parameter enum lock_mode mode, which must
be LOCK_X or LOCK_S.

row_merge_drop_table(): Assert that n_mysql_handles_opened == 0.
Unconditionally drop the table.

ha_innobase::add_index(): Acquire an X or S lock on the table, as appropriate.
After acquiring an X lock, assert that n_mysql_handles_opened == 1.
Remove the comments about dropping tables in the background.

ha_innobase::final_drop_index(): Acquire an X lock on the table.

dict_table_t: Remove version_number, to_be_dropped, and prebuilts.
ins_node_t: Remove table_version_number.

enum lock_mode: Move the definition from lock0lock.h to lock0types.h.

ROW_PREBUILT_OBSOLETE, row_update_prebuilt(), row_prebuilt_table_obsolete():
Remove.

row_prebuilt_t: Remove the declaration from row0types.h.

row_drop_table_for_mysql_no_commit(): Always print a warning if a table
was added to the background drop queue.