UNICAST3: create receiver table when non-first message is received first

• A and B
• B sends 2 messages to A: B1 and B2
• A receives B2 before B1 (both B1 and B2 are OOB)
• The current code has A drop B2 and B1 and send a SEND_FIRST_SEQNO message to B
• B resends B1, but not B2
• Retransmission needs to kick in before A receives B2. This might take up to xmit_interval * 2 ms before B2 is retransmitted and delivered

Scenario (JGRP-2293):

• A installs a new view
• B sends a LEAVE-REQ to A (B is leaving, too) on the view installation and a VIEW-ACK for the view. Both messages are unicasts to A and OOB
• The VIEW-ACK (B2) is received first and dropped, so it will have to be retransmitted
• This delays the view installation, as A waits for view_ack_collection_timeout ms until it has received all VIEW-ACKs

Workaround

• Set GMS.leave_timeout to a higher value (say 8000ms)

Solution

• When B2 is received, and it is not the first message and we don't have a receiver table for B yet, investigate whether we can create the receiver table anyway
• However, this requires the first seqno from B to always be 0
  --> Investigate whether the first seqno in UNICAST3 is always 0, then this solution will work