Just rename index in data dictionary and in InnoDB cache when it's possible.
Introduce ALTER_INDEX_RENAME for that purpose so that engines can optimize
such operation.
Unused code between macro MYSQL_RENAME_INDEX was removed.
compare_keys_but_name(): compare index definitions except for index names
Alter_inplace_info::rename_keys:
ha_innobase_inplace_ctx::rename_keys: vector of rename indexes
fill_alter_inplace_info():: fills Alter_inplace_info::rename_keys
