Set Zombie lastTick at start of drowned conversion

Fixes GH-1887

Prior to this change, the lastTick value for zombies would be set to
their initial spawn tick. This caused the drowned conversion process to
immediately occur rather than observing the vanilla delay.

Easy fix, just make sure its set again when the process actually starts.

And no, this change has absolutely nothing to do with the other thing.
That's been fixed and remains fixed.

Spigot-Server-Patches/Set-Zombie-last-tick-at-start-of-drowning-process.patch

@@ -0,0 +1,20 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Zach Brown <1254957+zachbr@users.noreply.github.com>
+Date: Mon, 4 Mar 2019 02:23:28 -0500
+Subject: [PATCH] Set Zombie last tick at start of drowning process
+
+Fixes GH-1887
+
+diff --git a/src/main/java/net/minecraft/server/EntityZombie.java b/src/main/java/net/minecraft/server/EntityZombie.java
+index 24bc3e0f0..bf2bed002 100644
+--- a/src/main/java/net/minecraft/server/EntityZombie.java
++++ b/src/main/java/net/minecraft/server/EntityZombie.java
+@@ -0,0 +0,0 @@ public class EntityZombie extends EntityMonster {
+                     ++this.bI;
+                     if (this.bI >= 600) {
+                         this.startDrownedConversion(300);
++                        this.lastTick = MinecraftServer.currentTick; // Paper - Make sure this is set at start of process - GH-1887
+                     }
+                 } else {
+                     this.bI = -1;
+--