res_xmpp: Use incremental backoff when a read error occurs

If a read error occurs, we immediately attempt a reconnect without any
delay. Instead, let's sleep and backoff up to 60 seconds before we try
again.

ASTERISK-24712 #close
Reported by: Matthias Urlichs

Change-Id: I6fe10ef4734837727437beab715e336777f13f48