Add Sender and Recipient hooks

[smol-ninja]

^{This is a copy of sablier-labs/flow#45 as I am unable to transfer the issue.}

As discussed in sablier-labs/flow#33, add the following hooks to the Flow protocol:

• onStreamWithdrawn on both Sender and Recipient
• onStreamPause on Recipient
• onStreamRefund on Recipient
• onAdjustRarePerSecond on Recipient

Comments

Andrei: I would suggest (to be more efficient) to implement the hooks at the end of the timeline (after we implement all the other features). I am saying this because we know from lockup, refactoring/modifying the tests of the hook functions is super time consuming. Do you agree?

Shub: I agree its not priority 1.

Andrei: As discussed privately, the hooks here are much more complicated than those in the lockup contract due to the debt. Also, since we’ve changed their design in lockup, we propose to put this on hold for the first release of Flow and see if there is a request for it. Temporarily locking this issue.