prevent appends except when annex.alwayscompact=false

prevent appends except when annex.alwayscompact=false

I would like for a new repo version to enable appends, but to do so
safely would need a v11 followed by a 1 year delay followed by a v12
that does it. Since a similar v9 and v10 transition is currently
happening, and is less than 6 months along in most repos, it does not
feel wise to stack up another year-long transition behind that. What if
I need to hurry up a new repo version for some other change?

Added todo so I remember to make this change at some time when a v11
and probably v12 repo version do make sense.

Sponsored-by: Dartmouth College's DANDI project