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