Allow LAN RPCs to be SYNC while WAN RPCs to be ASYNC

This will allow for interesting WAN architectures where RPC messages to members within the same LAN would be sent synchronously and RPC messages to remote members across a WAN to be sent asynchronously. This will allow for a cluster to span 2 data centres, (configured using TCP, for example), while allowing for some backups to be in the same data centre while others reside in a remote data centre.

From an email thread:

"So even if you have 4 owners, 2 in each DC, the "local" replication,
there will be no way to write to the local backup synchronously and
the remote backups asynchronously. I.e., either all comms are sync or
all comms are async.

Maybe this is something we can add in Infinispan. I.e., with node
hints (https://jira.jboss.org/browse/ISPN-180) we could detect
which recipients are local and which are remote, and accordingly split
the RPC into 2: a sync local RPC and an async remote RPC.

Node hinting isn't strictly necessary; the naming convention you
mentioned earlier would work in this regard as well, although I think
these hints is probably a better universal solution since we need this
for other things anyway."