Improve the way Vault lookups for offline players are handled (#1150)

1) The behaviour of online player lookups is unchanged - this only affects offline player data lookups.
2) If a 'offline player' request is made from an async task, it is executed normally. (the data is loaded)
3) If a 'offline player' request that cannot be immediately fulfilled is made from the main server thread, an exception is thrown.

The exception thrown as a result of 3) can be disabled in the config. In this case, the query is performed anyway. The main server thread will block until the request is complete.

common/src/main/java/me/lucko/luckperms/common/config/ConfigKeys.java

@@ -321,6 +321,11 @@ public final class ConfigKeys {
      */
     public static final ConfigKey<Boolean> COMMANDS_ALLOW_OP = EnduringKey.wrap(BooleanKey.of("commands-allow-op", true));
 
+    /**
+     * If Vault lookups for offline players on the main server thread should be enabled
+     */
+    public static final ConfigKey<Boolean> VAULT_UNSAFE_LOOKUPS = BooleanKey.of("vault-unsafe-lookups", false);
+
     /**
      * If the vault server option should be used
      */