need some flag for autotest

[asiroliu]

flag_1: pause incr replication

1. pause incr replication before incr replication start
2. do not block BinlogRelay
3. print in dtle log ,makesure the user knows that the flag is in effect

flag_2: continue incr replication

1. continue incr replication after set this flag or remove flag_1(pause incr replication)
2. print in dtle log ,makesure the user knows that the flag is in effect