Calculate the appropriate Coordinator IP address

[karya0]

Currently, the coordinator uses a mix ot hackery and tricks to compute its IP address. This involves a call to gethostname() followed by a call to getaddrinfo(). This is still unreliable and can give us 127.0.0.1 as the IP address, when we actually need the non-loopback IP.

[jiajuncao]

@karya0 127.0.0.1 is given in the scenario where dmtcp_launch has no --host parameter. At this point, SharedData is not initialized. If ENV_VAR_NAME_HOST is not set, hostname is set to DEFAULT_HOST, which is 127.0.0.1. _coordinatorSocket is then created using this address, which is wrong. This is harmless when running a local computation. But for a distributed one, remote processes won't connect to the correct coordinator.