Unified names for buffer timestamp meta

Submitted by Linus Svensson

Description

I'm currently adding support for wallclock timestamps on a proprietary source element of ours. I thought it's a good idea to have a common description for wallclock/utc timestamps regardless of what element produces them.

I file this bug to open up for a discussion/conclusion on names for both wallclock and UTC buffer timestamps meta.

I suggest something like:
timestamp/x-utc
timestamp/x-wallclock

It would be very nice if all elements that (potentially) adds wallclock timestamps on buffers can be accessed the same way in downstream elements.