Send cancel to peer when Session.GetBlock() is cancelled

[dirkmc]

If a client calls Session.GetBlock() or Session.GetBlocks() and then cancels the context, and the want has been sent to the peer, we should send a cancel to the peer.

Currently we only remove the want from the pending wants (ie wants that have not yet been sent to the peer).

Note that we need to make sure that no other session wants the same block before we send a cancel. We should be able to check that by asking the SessionInterestManager.

[Stebalien]

Could we reference count wants in the local wantlist?

[Stebalien]

Ah, nevermind. The interest cache is ref-counted anyways.