A quick look over what was changed:
CENTRAL_LOCK: lock owners can be node rather than individual threads[https://issues.jboss.org/browse/JGRP-1886]
Added an option to make the node the lock owner rather than the combo node:thread. This was needed by the JGroups clustering plugin for vert.x.
RSVP: option to not block caller[https://issues.jboss.org/browse/JGRP-1851]
This way, a caller knows that its message will get resent, but it doesn't have to block. Also added code to skip RSVP if the message was unicast and UNICAST3 was used as protocol (only used for multicast messages).
Docker image for JGroups[https://issues.jboss.org/browse/JGRP-1840]
mcast: new multicast diagnosis tool[https://issues.jboss.org/browse/JGRP-1832]
UNICAST3 / NAKACK2: limit the max size of retransmission requests[https://issues.jboss.org/browse/JGRP-1868]
When we have a retransmission request in UNICAST3 or NAKACK2 for a large number of messages, then the size of the retransmit message may become larger than what the transport can handle, and is therefore dropped. This leads to endless retransmissions.
The fix here is to retransmit only the oldest N messages such that the retransmit message doesn't exceed the max bundling size of the transport and to use a bitmap to identify missing messages to be retransmitted.
Also, the bitmaps used by SeqnoList reduce the size of a retransmission message.
Channel creation has unneeded synchronization[https://issues.jboss.org/browse/JGRP-1887]
Slowing down parallel creation of many channels; this was removed.
UNICAST3: sync receiver table with sender table[https://issues.jboss.org/browse/JGRP-1875]
In some edge cases, a receiver would not sync correctly with a sender.
JMX: unregistering a channel which didn't register any protocols issues warnings[https://issues.jboss.org/browse/JGRP-1894]
UDP: ip_ttl was ignored and is always 1[https://issues.jboss.org/browse/JGRP-1880]
MERGE3: in some cases, the information about subgroups is incorrect[https://issues.jboss.org/browse/JGRP-1876]
The actual MergeView was always correct, but the information about subgroups wasn't.
RELAY2 doesn't work with FORK[https://issues.jboss.org/browse/JGRP-1875]