p2p: do not destroy current ConnectedTurnTransport when a new file is incoming

When a new file is incoming, onTurnPeerConnection is triggered and the previous
turn endpoint deleted. This is dangerous if an user receives multiple files at
the same time. turn_ep_ should keep previous endpoints to allow incoming
files from multiple contacts.

Change-Id: Ia2c5b55bcc491a7d65936d27115b31809e6200ce