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

doc/bugs/initial_get_of_unlocked_file_fails_to_populate_pointer.mdwn

@@ -43,3 +43,43 @@ This outputs 1 for foo, followed by annex pointer files for files bar and baz.
 
 The previous fix attempt did make foo get populated, before that none
 of the files were populated.
+
+----
+
+`GIT_TRACE=1` shows that git only runs the smudge filter on the first
+file, not the other two. And indeed, restagePointerFile is only called
+on the first file.
+
+Added debugging to Database.Keys.reconcileStaged, and it adds all 3 files to
+the associated files table, but only adds the inode cache of foo.
+And that's what I see in the db after the fact too. Which is
+not itself a problem, to the extent that the other files are not
+populated, and only populated files have an inode cache recorded.
+
+So, Database.Keys.reconcileStaged is called after it gets foo,
+but before the other files are present, and in reconcilepointerfile it
+calls populatePointerFile and records the inode cache for foo.
+That is how foo gets populated.
+
+But, the other 2 files do not have populatePointerFile run on them.
+In moveAnnex, it calls getAssociatedFiles and somehow that returns
+`[]`, for all 3 files. This does not matter for foo, because it gets
+populated by reconcileStaged as explained above. But for the other 2, with
+no known associated files of course it fails to populate them.
+
+So: Why is getAssociatedFiles returning `[]`? Those calls come
+after Database.Keys.reconcileStaged has added the associated files,
+but are somehow not seeing the changes it made.
+
+Ah.. The keys db is opened in MultiWriter mode. 
+See the comment above the definition of MultiWriter,
+which explains that a write to a MultiWriter database,
+followed by a flushDbQueue may not be visible when reading
+from that same database.
+
+Verified this by making it re-open the db after reconcileStaged,
+which did fix the problem.
+
+A better fix is possible: Make MultiWriter mode not have this hidden
+gotcha, by re-opening the db after writing to it always. [[done]]
+--[[Joey]]