swarm: compress avatar to send in conversation's request

As it's sent over the DHT, the max value is 64k, also, even if sent
as a SIP message (if member was already connected and we add a new
non 1:1 swarm with this contact), the sendSIPMessage will fail due
to a too large message (PJSIP_EMSGTOOLONG).

Change-Id: I7908809fe2c3f6e08f9a4b9c6f8aac654ab86c42