Adds INSTANCE_ID header in all outgoing messages. HIAAProcessor uses that.

HeartbeatIAmAliveProcessor relies on incoming message's URI to determine the
 originating InstanceId. That is error prone and unnatural, since every
 layer but the network should not care about URIs. This commit ensures
 that all outgoing messages have an INSTANCE_ID header with the sending
 instance's ID.
This commit also ensures that HIAAProcessor reverts to looking at
 FROM if the INSTANCE_ID header is not present, since we need to
 make sure that during rolling upgrades from 1.9.6 forward we don't
 loose contact with older version instances
Merge pull request #1970 from digitalstain/1.9-maint

Adds INSTANCE_ID header in all outgoing messages. HIAAProcessor uses that