Folly::FutureDAG <-> Gossit

Folly::FutureDAG <-> Gossit

Summary:
Implements remove(), state_clean(), and reset() functions in order to allow for static FutureDAGS that can be modified in place and executed multiple times.

remove() removes the given handle from the nodes vector and cleans up all dependencies associated with it. Because of the way Handles are implemented, all Handles greater than the one removed are decremented (and therefore must be accounted for in the client-code). Current best-practice would be to remove nodes by most-recently added.

state_clean() removes the sink/source nodes added by go().

reset() removes all nodes but the top-level source node and resets dependency properties.

Reviewed By: tjkswaine

Differential Revision: D3486947

fbshipit-source-id: c8b9db6a139ee5b36aae6e9366c9b338cc49ede1