session: minimize number of tx events
session: minimize number of tx events

Type: improvement

Unset fifo tx event only if all data has been dequeued. Avoids frequent
re-scheduling of sessions as new sessions.

Signed-off-by: Florin Coras <c9715e5cdf0718c2e9b10678757dfb027875eba2@cisco.com>
Change-Id: I36a4c90b97e0255b88782032fb029640e6a73e90

chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp