Make DDL operations that involve FULLTEXT INDEX atomic.
In particular, we must drop the internal FTS_ tables in the same
DDL transaction with ALTER TABLE.

Remove all references to fts_drop_orphaned_tables().

row_merge_drop_temp_indexes(): Drop also the internal FTS_ tables
that are associated with index stubs that were created in
prepare_inplace_alter_table_dict() for
CREATE FULLTEXT INDEX before the server was killed.

fts_clear_all(): Remove the fts_drop_tables() call. It has to be
executed before the transaction is committed!

dict_load_indexes(): Do not load any metadata for index stubs
that had been created by prepare_inplace_alter_table_dict()

fts_create_one_common_table(), fts_create_common_tables(),
fts_create_one_index_table(), fts_create_index_tables():
Remove redundant error handling. The tables will be dropped
just fine by dict_drop_index_tree().

commit_try_norebuild(): Also drop the FTS_ tables when dropping
FULLTEXT INDEX.

The changes to the test case innodb_fts.crash_recovery has been
extensively tested. The non-debug server will be killed while
the 3 ALTER TABLE are in any phase of execution. With the debug
server, DEBUG_SYNC should make the test deterministic.