alter table comment rebuild whole table

alter table comment cause tokudb to rebuild the whole table, which is not needed, only FRM metadata should be changed.

create table t1(id int) engine=TokuDB;
insert into t1 values (1), (2), (3);
alter table t1 comment='test table';

gdb which the alter statement, we can see that `HA_ALTER_INPLACE_NOT_SUPPORTED` is returned from `ha_tokudb::check_if_supported_inplace_alter` checking and table copy is used as:

```
#0 toku_ft_root_put_msg (ft=0x7fa79d296600, msg=..., gc_info=0x7fa842650400) at percona-server/storage/tokudb/PerconaFT/ft/ft-ops.cc:1917
#1 0x00000000017bdddb in toku_ft_send_insert (ft_handle=0x7fa799e120c0, key=0x7fa8426506b0, val=0x7fa8426506d0, xids=0x7fa79d216030, type=FT_INSERT, gc_info=0x7fa842650400) at percona-server/storage/tokudb/PerconaFT/ft/ft-ops.cc:2557
#2 0x00000000017bcf25 in toku_ft_maybe_insert (ft_h=0x7fa799e120c0, key=0x7fa8426506b0, val=0x7fa8426506d0, txn=0x7fa79d296000, oplsn_valid=false, oplsn=..., do_logging=true, type=FT_INSERT) at percona-server/storage/tokudb/PerconaFT/ft/ft-ops.cc:2442
#3 0x00000000016d6c9a in db_put (db=0x7fa79d29cfc0, txn=0x7fa79d295000, key=0x7fa8426506b0, val=0x7fa8426506d0, flags=0, do_log=true) at percona-server/storage/tokudb/PerconaFT/src/ydb_write.cc:221
#4 0x00000000016d6fe3 in toku_db_put (db=0x7fa79d29cfc0, txn=0x7fa79d295000, key=0x7fa8426506b0, val=0x7fa8426506d0, flags=0, holds_mo_lock=false) at percona-server/storage/tokudb/PerconaFT/src/ydb_write.cc:248
#5 0x00000000016dbbdc in autotxn_db_put (db=0x7fa79d29cfc0, txn=0x7fa79d295000, key=0x7fa8426506b0, data=0x7fa8426506d0, flags=0) at percona-server/storage/tokudb/PerconaFT/src/ydb_write.cc:1078
#6 0x000000000164fa94 in ha_tokudb::insert_row_to_main_dictionary (this=0x7fa79d258810, record=0x7fa79d24d010 "\375\001", pk_key=0x7fa8426506b0, pk_val=0x7fa8426506d0, txn=0x7fa79d295000) at percona-server/storage/tokudb/ha_tokudb.cc:3882
#7 0x0000000001650d6e in ha_tokudb::write_row (this=0x7fa79d258810, record=0x7fa79d24d010 "\375\001") at percona-server/storage/tokudb/ha_tokudb.cc:4093
#8 0x00000000006e8a57 in handler::ha_write_row (this=0x7fa79d258810, buf=0x7fa79d24d010 "\375\001") at percona-server/sql/handler.cc:7921
#9 0x0000000000af2061 in copy_data_between_tables (from=0x7fa79d264000, to=0x7fa79d2df000, create=..., ignore=false, order_num=0, order=0x0, copied=0x7fa8426527d8, deleted=0x7fa8426527d0, keys_onoff=Alter_info::LEAVE_AS_IS, alter_ctx=0x7fa842651d00) at percona-server/sql/sql_table.cc:9652
#10 0x0000000000aef105 in mysql_alter_table (thd=0x7fa83744b700, new_db=0x7fa79d224668 "test", new_name=0x0, create_info=0x7fa842652bc0, table_list=0x7fa79d224108, alter_info=0x7fa842652b00, order_num=0, order=0x0, ignore=false) at percona-server/sql/sql_table.cc:9180
#11 0x0000000000d625c3 in Sql_cmd_alter_table::execute (this=0x7fa79d224680, thd=0x7fa83744b700) at percona-server/sql/sql_alter.cc:335
#12 0x0000000000a0675c in mysql_execute_command (thd=0x7fa83744b700) at percona-server/sql/sql_parse.cc:5588
#13 0x0000000000a0dd37 in mysql_parse (thd=0x7fa83744b700, rawbuf=0x7fa79d224010 "alter table t1 comment='test table'", length=35, parser_state=0x7fa8426544c0) at percona-server/sql/sql_parse.cc:7080
#14 0x00000000009ef398 in dispatch_command (command=COM_QUERY, thd=0x7fa83744b700, packet=0x7fa837455081 "alter table t1 comment='test table'", packet_length=35) at percona-server/sql/sql_parse.cc:1487
#15 0x00000000009ecda7 in do_command (thd=0x7fa83744b700) at percona-server/sql/sql_parse.cc:1062
#16 0x00000000009814d8 in do_handle_one_connection (thd_arg=0x7fa83744b700) at percona-server/sql/sql_connect.cc:1590
#17 0x0000000000980e9b in handle_one_connection (arg=0x7fa83744b700) at percona-server/sql/sql_connect.cc:1494
#18 0x000000000103d8fd in pfs_spawn_thread (arg=0x7fa83c3ee260) at percona-server/storage/perfschema/pfs.cc:1861
#19 0x00007fa842249aa1 in start_thread () from /lib64/libpthread.so.0
#20 0x00007fa840fb293d in clone () from /lib64/libc.so.6
```