TS-1713: SRV support refine

the currenty SRV supporting in the DNS is not complete, and here is
a full refine of the old codes, as a side affect we also bump the
HostDB database version to 3.0, and increase the disk spaces from
32MB to 200MB:
CONFIG proxy.config.hostdb.storage_size INT 200M

*******CAUTION******
YOU ARE WARNED THAT FAIL TO ADJUST THE hostdb.storage_size CAN NOT
START THE SERVER PROCESS PROPERLY!!
TS-1796  remove cluster connection number change handle