implement "pubsub numsub"; fix glitch in command tree (impacts "command" and "command info" - gave nil name)

only need to take the write lock if the connection has subscriptions; don't need to impact regular connections

lib updates (in particular, see https://github.com/aspnet/KestrelHttpServer/issues/2694#issuecomment-420347853)

* make transaction inner tasks use RunContinuationsAsynchronously (for discussion, see issue #943); detect RunContinuationsAsynchronously in ResultBox completion, and set directly rather than deferred