Provide more convenient ALTER SERVICE as an alternative to LINK/UNLINK

The current maxctrl link and maxctrl unlink commands are useful for adding a single server to an existing service or monitor. If the whole set of servers needs to be defined, a more convenient way to do it would be to pass it as maxctrl alter service MyService targets=server1,server2,server3.

This can be implemented completely on the client side and no changes need to be done in MaxScale.