Disallow old version of VimFx to communicate with new version

Disallow old version of VimFx to communicate with new version

Commit 865fdaba attempted to fix an issue where frame script message listeners
from the old version of VimFx wasn't shut down properly when VimFx was updated.

Since then, the problem has become very rare, but unfortunately still happens
occasionally. I really have no idea why it can still happen. It might even be
that there's nothing that can be done due to the way Firefox works.

This commit tries to make any leftover frame script message listeners useless,
by adding the `BUILD_TIME` in all message names.