Track half-closed stream IDs

In order to eliminate a side channel attack described in https://petsymposium.org/2018/files/papers/issue2/popets-2018-0011.pdf ("DropMark" attack) we need a way to determine if a stream id is invalid.

Many clients (particularly Firefox) will hang up on streams that still have data in flight. In this case, Tor clients send RELAY_COMMAND_END when they are done with a stream, and immediately remove that stream ID from their valid stream mapping. The remaining application data continues to arrive, but is silently dropped by the Tor client. The result is that this ignored stream data currently can't be distinguished from injected dummy traffic with completely random stream IDs, and this fact can be used to mount side channel attacks.

A similar situation exists for spurious RELAY_ENDs.