vnet: Add field for transmit time
vnet: Add field for transmit time

Add a field in the buffer header for transmit time. High-level protocols
need a consistent way to do this.

Change-Id: Ifff60ee5e32eed7aa6f6ae8e6fb1dd9d7870e9ee
Signed-off-by: Kevin Paul Herbert <c627a7b38bc4e8ca26b3221b5490fd785ff4f7ed@cisco.com>

milanlenco/vpp,chrisy/vpp,muharif/vpp,chrisy/vpp,chrisy/vpp,milanlenco/vpp,GabrielGanne/vpp-flowtable,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,GabrielGanne/vpp-flowtable,chrisy/vpp,milanlenco/vpp,vpp-dev/vpp,chrisy/vpp,muharif/vpp,vpp-dev/vpp,vpp-dev/vpp,GabrielGanne/vpp-flowtable,milanlenco/vpp,muharif/vpp,FDio/vpp,muharif/vpp,milanlenco/vpp,milanlenco/vpp,GabrielGanne/vpp-flowtable,muharif/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,GabrielGanne/vpp-flowtable,FDio/vpp,muharif/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,GabrielGanne/vpp-flowtable,FDio/vpp,FDio/vpp