Don't wait for pending events to process on startup

Previously, on large Gerrit installations with many projects and/or many
replication destinations, the replication plugin could take very long
periods of time to startup. This was particularly a problem if the
pending(persisted) event count was large as they all were rescheduled
before the plugin finished initializing. Change this behavior so that
startup merely begins the process of scheduling the pending events, but
does not wait for them to complete.

Bug: Issue 12769
Change-Id: I224c2ce2a35f987af2343089b9bb00a7fcb7e3be

src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java
index e704978..f2d8ec5 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java
@@ -99,7 +99,9 @@
     if (!running) {
       config.startup(workQueue);
       running = true;
-      firePendingEvents();
+      Thread t = new Thread(this::firePendingEvents, "firePendingEvents");
+      t.setDaemon(true);
+      t.start();
       fireBeforeStartupEvents();
     }
   }
@@ -216,6 +218,8 @@
           repLog.error("Encountered malformed URI for persisted event %s", t);
         }
       }
+    } catch (Throwable e) {
+      repLog.error("Unexpected error while firing pending events", e);
     } finally {
       replaying = false;
     }