Cluster: claim ping_sent time even if we can't connect.

This fixes a potential bug that was never observed in practice since
what happens is that the asynchronous connect returns ok (to fail later,
calling the handler) every time, so a ping is queued, and sent_ping
happens to always be populated.

Howver technically connect(2) with a non blocking socket may return an
error synchronously, so before this fix the code was not correct.
Unix domain sockets incorrectly listed as IPv6.
Merge pull request #2010 from yossigo/unixdomain-info-string

Unix domain sockets incorrectly listed as IPv6.
anetPeerToString() refactoring and more explicit checks.

Related to PR #2010.
anetPeerToString(): make unix sockets distinguishable.

Following the CLIENT LIST output format, we prefix the unix socket
address with a "/" so that it is different than an IPv4/6 address.
This makes parsing simpler.

Related to #2010.