Trunk nightly users: New MUC storage format

[Matthew Wild]

Hello,

This post is for users of trunk nightly builds.

Starting from tomorrow's trunk nightly build (2020-03-13), Prosody
will now store information about MUCs differently[1].

For all normal deployments, there is nothing you need to do. It is an
invisible internal change.

There are just two things to be aware of:

1) The new format is not compatible with Prosody 0.10 and earlier,
so you will not be able to downgrade (0.11 works fine).

2) If you are a developer accessing Prosody's data store directly
(on disk or via SQL) to get information about MUC rooms instead of
using an API, your code may break.

If for any reason you want to remain on the current storage format,
simply set `new_muc_storage_format = false` in your config file
(global section or directly under your MUC component) before you
upgrade.

Regards,
Matthew

[1]: https://hg.prosody.im/trunk/rev/83668e16b9a3