Merge changes from topic "tcp_keepalive_fix"

* changes:
  Use API TcpKeepalivePacketData in ClientModeImpl
  Add TcpKeepalivePacketData to SystemApi
tree: a999c893050ec56e0adae1e1f14af516bf2cebc4
  1. core/
  2. services/
  3. tests/
  4. OWNERS