Use System.nanoTime() to initialize Engine.lastWriteNanos

On _lastWriteNanos_ we use System.nanoTime() to initialize this since:
 * we use the value for figuring out if the shard / engine is active so if we startup and no write has happened yet we still consider it active
   for the duration of the configured active to inactive period. If we initialize to 0 or Long.MAX_VALUE we either immediately or never mark it
   inactive if no writes at all happen to the shard.
 * we also use this to flush big-ass merges on an inactive engine / shard but if we we initialize 0 or Long.MAX_VALUE we either immediately or never
   commit merges even though we shouldn't from a user perspective (this can also have funky sideeffects in tests when we open indices with lots of segments
   and suddenly merges kick in.

core/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -80,7 +80,18 @@ public abstract class Engine implements Closeable {
     protected final ReleasableLock readLock = new ReleasableLock(rwl.readLock());
     protected final ReleasableLock writeLock = new ReleasableLock(rwl.writeLock());
     protected volatile Throwable failedEngine = null;
-    protected volatile long lastWriteNanos = Long.MAX_VALUE; // no write yet!
+    /*
+     * on <tt>lastWriteNanos</tt> we use System.nanoTime() to initialize this since:
+     *  - we use the value for figuring out if the shard / engine is active so if we startup and no write has happened yet we still consider it active
+     *    for the duration of the configured active to inactive period. If we initialize to 0 or Long.MAX_VALUE we either immediately or never mark it
+     *    inactive if no writes at all happen to the shard.
+     *  - we also use this to flush big-ass merges on an inactive engine / shard but if we we initialize 0 or Long.MAX_VALUE we either immediately or never
+     *    commit merges even though we shouldn't from a user perspective (this can also have funky sideeffects in tests when we open indices with lots of segments
+     *    and suddenly merges kick in.
+     *  NOTE: don't use this value for anything accurate it's a best effort for freeing up diskspace after merges and on a shard level to reduce index buffer sizes on
+     *  inactive shards.
+     */
+    protected volatile long lastWriteNanos = System.nanoTime();
 
     protected Engine(EngineConfig engineConfig) {
         Objects.requireNonNull(engineConfig.getStore(), "Store must be provided to the engine");