=act #15991 delay sending of ACK until we have updated pendingWrite

Otherwise, the user-level actor may already have sent the next chunk before
the pendingWrite has been updated and the TcpConnection will reject it.
(cherry picked from commit 96758e6)
Merge pull request #16267 from drewhk/wip-15991-tcp-pendingwrite-fix-forward-port-drewhk

=act #15991 delay sending of ACK until .. (forward port)