This deadlock would happen when calling SessionsForBlock (holding
bitswap.sessLk) while the session's main loop was trying to deregister the
session (taking bitswap.sessLk).

I've also defensively added selects on contexts for two other channel writes
just in case.

fixes #4394

...well, it fixes *a* deadlock showing up in that issue, there may be more.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>