Prototype: Directly call parent node's function

[nikhil96sher]

Initial prototype of directly calling parent node's function instead of passing data through channels.

Things currently working:

1. Can directly call parent node's function instead of passing data through channel.
2. Can perform this with multiple downstream nodes in parallel.

Things to add:

1. Eager/Lazy option when creating ExecutionNode. Eager execution node starts the execution in another thread. Lazy execution node doesn't start any execution.
2. Interior Mutability while keeping ExecutionNode immutably borrowed. Important because the same node needs to be borrowed in child nodes. So, we cannot have it borrowed mutably.

[mIXs222]

Few things i saw during meeting

[mIXs222]

node: Weak<RwLock<ExecutionNode>>, ?

Then we can check during upgrade like match node.upgrade() {Some(...) => ..., None => ...}

[mIXs222]

node: Weak<RwLock<ExecutionNode>>, ?

[nikhil96sher]

I think we can get rid of the Option.
I initially added it to first create operation without a node. Then create the execution node with this operation while updating the node in the operation.
But Weak itself should be good enough to support that.

[mIXs222]

Just return self.input_data.get(partition_num) ?

[nikhil96sher]

So, turns out .get() on a Vec<Item> returns Option<&Item>.
Directly returning the value mismatches the signature (Option<i64> vs Option<&i64>). That's why did the reconstruction.
But there should be some better way.