BUG/MEDIUM: stream: never allocate connection addresses from signal handler

BUG/MEDIUM: stream: never allocate connection addresses from signal handler

In __strm_dump_to_buffer(), we call conn_get_src()/conn_get_dst() to try
to retrieve the connection's IP addresses. But this function may be called
from a signal handler to dump a currently running stream, and if the
addresses were not allocated yet, a poll_alloc() will be performed while
we might possibly already be running pools code, resulting in pool list
corruption.

Let's just make sure we don't call these sensitive functions there when
called from a signal handler.

This must be backported at least to 3.1 and ideally all other versions,
along with this previous commit:

  MINOR: tinfo: add a new thread flag to indicate a call from a sig handler