h264parse/h265parse: Various fixes around alignment=nal and latency

The main focus for this patchset was to allow low latency parsing. So this patchset notably fixes nal alignment parsing, removes uneeded latency, honor the MARKER bit, etc. It also comes with many new tests which should cover most of the changes Fixes #800 (closed)

cc @gkiagia

• Depends on !1193 (merged)
• Depends on gst-integration-testsuites!65 (merged)