Decide how to handle control port events for IPv6 reachability self-tests

The control spec has two reachability self-test events.

Here is how they are specified:

     CHECKING_REACHABILITY
     "ORADDRESS=IP:port"
     "DIRADDRESS=IP:port"
       We're going to start testing the reachability of our external OR port
       or directory port.

     REACHABILITY_SUCCEEDED
     "ORADDRESS=IP:port"
     "DIRADDRESS=IP:port"
       We successfully verified the reachability of our external OR port or
       directory port (depending on which of ORADDRESS or DIRADDRESS is
       given.)

And here is what tor actually sends:

CHECKING_REACHABILITY ORADDRESS=IPv4:port
CHECKING_REACHABILITY DIRADDRESS=IPv4:port
REACHABILITY_SUCCEEDED ORADDRESS=IPv4:port
REACHABILITY_SUCCEEDED DIRADDRESS=IPv4:port

When we add IPv6 reachability events, we could break some (buggy) control parsers with:

CHECKING_REACHABILITY ORADDRESS=[IPv6]:port
REACHABILITY_SUCCEEDED ORADDRESS=[IPv6]:port

How should we handle this change?