sio: optimize sio_strfaddr() for the most common case

SIO library provides a wrapper for getnameinfo able to stringify
Unix socket addresses. But it does not care about limited
Tarantool stack and allocates buffers for getnameinfo() right on
it - ~1Kb. Besides, after successful getnameinfo() the result is
copied onto another static buffer.

This patch optimizes sio_strfaddr() for the most common case -
AF_INET, when 32 bytes is more than enough for any IP:Port pair,
and writes the result into the target buffer directly.

The main motivation behind this commit is that SWIM makes active
use of sio_strfaddr() for logging - for each received/sent
message it writes a couple of addresses into a log. It does it in
verbose mode, but the say() function arguments are still
calculated even when the active mode is lower than verbose.