Plugin limits to 5 TPS despite players being connected

[aRestless]

Went through our telemetry and noticed a case where a user was connected to the server (according to Prometheus data and console output) but the plugin still limited the server to 5 TPS.

After reviewing the code it seems like the only explanation is that Universe.get().getPlayerCount() returns 0 while Universe.get().getDefaultWorld().getPlayerCount() would return 1, as this is the data source for the Prometheus plugin.

Fix: determine player count by summing up player counts across all worlds and maxing it against Universe.get().getPlayerCount().

[aRestless]

Important additional context: #8 (comment) (TL;DR: It's not as bad as was assumed first).