Move duplicated logic from main & worker user_syncing methods to
set_state, which is called direclty on main and over replication
for workers.

The new logic is driven by a is_sync flag, which avoids overriding
the "busy" presence state.

Building on the previous commit, last_user_sync_ts can now be set
based on the is_sync flag.

VALID_PRESENCE only contains the busy presence if it is enabled, thus
presence can only be equal to "busy" if it is already enabled.

See inline comments, but the user_syncing method already calls
set_state(...) to update the presence state, we don't need custom
logic for the offline -> online transition.

…esence