stream-level SENDME recv window accounting is incorrect with packing and fragmentation (prop340)

stream-level SENDME recv windows are currently updated in StreamReader::recv_raw, decrementing once per message. They should instead be decremented per cell, which won't be the same thing once packing and fragmentation are enabled (prop340).

In particular:

If an incoming cell contains two DATA messages for the same stream, the window should only be decremented once (pending clarification in torspec!265 (merged)). The current code will decrement it twice.
If we implement a message that both counts towards windows and can be fragmented (such as DATAGRAM), the window should be decremented once per cell, not just once for the whole message.

cc @dgoulet