Introduce new lockstep syncing system, modularize sync modes (#178)

* Start work on modular sync systems

* Add experimental lockstep sync system, close #69

* Refactor RedisMessageType enum

* Fixup lockstep syncing

* Bump to 3.1

* Update docs with details about the new Sync Modes

* Sync mode config key is `mode` instead of `type`

* Add server to data snapshot overview

* API: Add API for setting data syncers

* Fixup weird statistic matching logic

common/src/main/java/net/william278/husksync/util/DataSnapshotOverview.java

@@ -70,6 +70,8 @@ public class DataSnapshotOverview {
         }
         locales.getLocale("data_manager_cause", snapshot.getSaveCause().getDisplayName())
                 .ifPresent(user::sendMessage);
+        locales.getLocale("data_manager_server", snapshot.getServerName())
+                .ifPresent(user::sendMessage);
 
         // User status data, if present in the snapshot
         final Optional<Data.Health> health = snapshot.getHealth();