[CX] Refactor exception handling in LSMInsertDeleteOperatorNodePushable

The exception handling in LSMInsertDeleteOperatorNodePushable is a bit hard to follow.

– Feed requires the two classes work together (as producer-consumer) to handle duplicate-key-exceptions:  LSMInsertDeleteOperatorNodePushable and SyncFeedRuntimeInputHandler.

-- LSMInsertDeleteOperatorNodePushable has to keep track of the cursor of an incoming frame as its state.

There could be an alternative way for doing this:

– Pass an exception handler interface into LSMInsertDeleteOperatorNodePushable to determine either throw or swallow a duplicate-key-exception and then propagate tuples to the downstream accordingly.  In this way, LSMInsertDeleteOperatorNodePushable can be arbitrarily composable with other runtime operators, with configurable semantics.