make sure the very first fragment triggers an rx_callback
make sure the very first fragment triggers an rx_callback

On the first session server, the first payload packet gets queued
but the Rx callback will only be invoked when the second packet
is received.

By initializing enqueue_epoch in the session, we can make sure that
the RX callback will be invoked for the first packet as well.

Change-Id: I60e2cf8d4d2ca71d461ca003d459c8e6e6956a4d
Signed-off-by: Andreas Schultz <6963656cd5f1036a86f38d6c01e67c3f63074c58@warp10.net>

vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp