Restore in-memory index

[link2xt]

It was introduced in #632 and reverted in #643 because of the bug #640

We can restore it once chatmail/core#7005 is solved in the core.

[link2xt]

I am not sure this INDEX=MEMORY is a good idea. It creates a new index for each connection apparently. If we want to move it off the disk then maybe better move it to a tmpfs partition:
https://doc.dovecot.org/2.3/configuration_manual/mail_location/#mail-location-settings-index-files

A new tmpfs partition can be created with a systemd unit, see for example:

$ systemctl cat tmp.mount
# /usr/lib/systemd/system/tmp.mount
#  SPDX-License-Identifier: LGPL-2.1-or-later
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Temporary Directory /tmp
Documentation=https://systemd.io/TEMPORARY_DIRECTORIES
Documentation=man:file-hierarchy(7)
Documentation=https://systemd.io/API_FILE_SYSTEMS
ConditionPathIsSymbolicLink=!/tmp
DefaultDependencies=no
Conflicts=umount.target
Before=local-fs.target umount.target
After=swap.target

[Mount]
What=tmpfs
Where=/tmp
Type=tmpfs
Options=mode=1777,strictatime,nosuid,nodev,size=50%%,nr_inodes=1m,x-systemd.graceful-option=usrquota

Then it is still in memory, but at least shared. This will however likely become a problem for small server that have disk but not much memory.