finishing up move --from --to

finishing up move --from --to

Lock the local content for drop after getting it from src, to prevent another
process from using the local content as a copy and dropping it from src,
which would prevent dropping the local content after sending it to dest.

Support resuming an interrupted move that downloaded the content from
src, leaving the local content populated. In this case, the location log
has not been updated to say the content is present locally, so we can
assume that it's resuming and go ahead and drop the local content after
sending it to dest.

Note that if a `git-annex get` is being ran at the same time as a
`git-annex move --from --to`, it may get a file just before the move
processes it. So the location log has not been updated yet, and the move
thinks it's resuming. Resulting in local copy being dropped after it's
sent to the dest. This race is something we'll just have to live with,
it seems.

I also gave up on the idea of checking if the location log had been updated
by a `git-annex get` that is ran at the same time. That wouldn't work, because
the location log is precached in the seek stage, so reading it again after
sending the content to dest would not notice changes made to it, unless the cache
were invalidated, which would slow it down a lot. That idea anyway was subject
to races where it would not detect the concurrent `git-annex get`.

So concurrent `git-annex get` will have results that may be surprising.
To make that less surprising, updated the documentation of this feature to
be explicit that it downloads content to the local repository
temporarily.

Sponsored-by: Dartmouth College's DANDI project