feat: Store blobs in subdirs with random names (#4309)

Recently there was an accident with a chatbot that replaced its avatar set from the command line
with an unrelated avatar of a contact. Both the `selfavatar` setting and the contact avatar `i`
param pointed to `$BLOBDIR/avatar.png` at the time it was detected. How this happened is unclear,
but it is possible that `avatar.png` was removed, unmounted or otherwise not detected by the core,
and the core stored avatar received from the contact as `avatar.png`, while `selfavatar` config
still pointed to `$BLOBDIR/avatar.png`.

Such bugs are unavoidable even if the core itself has no bugs as we cannot rely on blobdir not
reside on the faulty network filesystem, being incorrectly backed up and restored etc., so we should
assume that files may be randomly removed. Then there may be dangling `$BLOBDIR/...` references in
the database which may accidentally point to unrelated files, could even be an `avatar.png` file
sent to the bot in private.

To prevent such bugs, store blobs in blobdir subdirs with random names. Also this helps when we
receive multiple attachments having the same name -- before, random filename suffixes were added to
subsequent attachments, now attachments preserve their filenames which is important if they are
opened in external programs.

python/tests/test_2_increation.py

@@ -50,8 +50,8 @@ class TestOnlineInCreation:
         src = tmp_path / "file.txt"
         src.write_text("hello there\n")
         msg = chat.send_file(str(src))
-        assert msg.filename.startswith(os.path.join(ac1.get_blobdir(), "file"))
-        assert msg.filename.endswith(".txt")
+        assert msg.filename.startswith(ac1.get_blobdir())
+        assert msg.filename.endswith("file.txt")
 
     def test_forward_increation(self, acfactory, data, lp):
         ac1, ac2 = acfactory.get_online_accounts(2)