tcp: handle start tx event in cubic
tcp: handle start tx event in cubic

If app was idle update start time of current congestion avoidance phase
unless tcp connection was not idle.

Type: improvement

Signed-off-by: Florin Coras <c9715e5cdf0718c2e9b10678757dfb027875eba2@cisco.com>
Change-Id: Idf6a03a9ef96c409462de9f9cb19df609f730afe

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