Not ready to tackle the Python2/Python3 monster a week before release.

We have settled on using 'Upgrading' to list changes like this, while
'Changes' is a free-form prose overview ("about this release").

Helps: #2764

Fixes:	#2773

The fact that the previous object was IMS/INM eligable does not make
one created from it eligable as well.

Also test for the miss + hfm case with IMS/INM

Fixes #2763

it really is about time we rename OC_F_PASS -> OC_F_HFM, but
my recent cleanup attempt included too much change before the release

constraints.

Related to: #2763

Fixes #2761

The only way H2_Send_Frame can fail is if the TCP connection is
dead, so reach out and set the session errored and be done with it,
rather than pass an inconvenient error status down through the callers.

While here use writev(2) instead of two write(2) calls.

Most of the heavy lifting by:	slink

Fixes: #2760, #2754, #2654

require new bytes to be received.