This is to address an issue in which `m.presence` results on initial
sync are not returning entries of users who are currently offline.
The original behaviour was from
https://github.com/element-hq/synapse/issues/1535
This change is useful for applications that use the
presence system for tracking user profile information/updates (e.g.
https://github.com/element-hq/synapse/pull/16992 or for profile status
messages).
This is gated behind a new configuration option to avoid performance
impact for applications that don't need this, as a pragmatic solution
for now.
