ENT-2659 Eliminate lots of contention in serialization (#4120)

* Remove most contentious bit of mutex in the codebase
Fix up SerializerFactory
Make getSerializerFactory() non-blocking
Workaround for DJVM issues

* Some clean up and also de-contend the RPC client.

* Fix up attachment class loader code.

* Bug fix

serialization/src/test/kotlin/net/corda/serialization/internal/amqp/StaticInitialisationOfSerializedObjectTest.kt

@@ -63,9 +63,9 @@ class StaticInitialisationOfSerializedObjectTest {
         // we can't actually construct one
         sf.get(null, D::class.java)
 
-        // post creation of the serializer we should have one element in the map, this
+        // post creation of the serializer we should have two elements in the map, this
         // proves we didn't statically construct an instance of C when building the serializer
-        assertEquals(1, serialisersByType.size)
+        assertEquals(2, serialisersByType.size)
     }
 
     @Test