improve sqlite MultiWriter handling of read after write

improve sqlite MultiWriter handling of read after write

This removes a messy caveat that was easy to forget and caused at least one
bug. The price paid is that, after a write to a MultiWriter db, it has to
close the db connection that it had been using to read, and open a new
connection. So it might be a little bit slower. But, writes are usually
batched together, so there's often only a single write, and so there should
not be much of a slowdown. Notice that SingleWriter already closed the db
connection after a write, so paid the same overhead.

This is the second try at fixing a bug: git-annex get when run as the first
git-annex command in a new repo did not populate all unlocked files.
(Reversion in version 8.20210621)

Sponsored-by: Boyd Stephen Smith Jr. on Patreon