codegen - create/delete handler pre execution check

[ammokhov]

Currently preExistanceCheck/preDeletionCheck invoke ReadHandler which returns sub chain with call graph but it doesnt get converged into the main chain. This might lead to an unstable handler behavior on reinvocation - scenario:

Step 1: CREATE Invoked
Step 1.1: check for pre existance - not found, execution continues
Step 1.2: create a resource and time is up - lambda exited

Step 2: CREATE Reinvoked
Step 2.1: check for pre existance - resource found (created from previous execution) - throws already exist exception.

In this case Create Handler fails to reach SUCCESS status.