reload @@config_all failed may lead to connections for new added datahost in config leaking

[irene-coming]

• dble version：
  5.6.29-dble-9.9.9.9-d8a5c79cc4efda58659d7990340fde23777a1558-20190903023556

• preconditions ：
  no

• configs：

schema.xml

rule.xml

server.xml

• steps：
  step1. start a trx to a sharding table, eg:

begin;
select * from sharding_2_t1 limit 2;

step2. alter the sharding table in another session gets blocked:

alter table sharding_2_t1 add column c int; -- blocked here

step3. modify the config schema.xml, to add a new datahost, and add table to use datanode in it

step4. query "reload @@config_all" to dble manager port, get failed for:
ERROR 1003 (HY000): Reload config failure.The reason is There is other session is doing DDL

step5. netstat -pent | grep {new added datahost ip} , there will be connections leaving there at "ESTABLISHED" state

• expect result：
  1. if reload @@config_all failed, the new added connections should be recyled
• real result：
  1.reload @@config_all failed may lead to connections leaking as example above
• supplements：
  1. to consider other possible reasons lead to reload failed

[yexiaoli88]

verified on version: 5.6.29-dble-9.9.9.9-77a301ab1dd9a58dbf8fad345dfb2d563a54ebc8-20190905021744