ha_innobase::prepare_inplace_alter_table(): If the clustered index is
corrupted or the table is marked as corrupted, refuse any ALTER TABLE
operation that would affect the InnoDB metadata.

prepare_inplace_alter_table_dict(): If the table is not being rebuilt,
and a corrupted secondary index would remain after the ALTER TABLE
operation, refuse the operation.

commit_try_norebuild(), row_merge_drop_indexes_dict():
Handle error==DB_TOO_MANY_CONCURRENT_TRXS as a special case.

RB: 7323
Reviewed-by: Kevin Lewis <kevin.lewis@oracle.com>
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>