add pre-shutdown action to be executed inside ChronicleHashCloseOnExitHook.

This is useful when we want to do an orderly shutdown.
For example: To carry out graceful shutdown and explicitly control when a ChronicleHash
is closed, a pre-shutdown action can be a wait on a CountDownLatch that would be released
appropriately.

src/main/java/net/openhft/chronicle/hash/impl/VanillaChronicleHash.java

@@ -125,6 +125,7 @@ public abstract class VanillaChronicleHash<K,
     public transient CompactOffHeapLinearHashTable hashLookup;
     public transient Identity identity;
     protected int log2TiersInBulk;
+    private Runnable preShutdownAction;
     /////////////////////////////////////////////////
     // Bytes Store (essentially, the base address) and serialization-dependent offsets
     protected transient BytesStore bs;
@@ -208,6 +209,8 @@ public abstract class VanillaChronicleHash<K,
         tierBulkSizeInBytes = computeTierBulkBytesSize(tiersInBulk);
 
         checksumEntries = privateAPI.checksumEntries();
+
+        preShutdownAction = privateAPI.getPreShutdownAction();
     }
 
     public static IOException throwRecoveryOrReturnIOException(
@@ -230,6 +233,10 @@ public abstract class VanillaChronicleHash<K,
         initTransients();
     }
 
+    public Runnable getPreShutdownAction() {
+        return preShutdownAction;
+    }
+
     protected void readMarshallableFields(@NotNull WireIn wireIn) {
         dataFileVersion = wireIn.read(() -> "dataFileVersion").text();