Calling `Connection::close()` should prevent any application data from being written

[close()](https://github.com/cloudflare/quiche/blob/master/quiche/src/lib.rs#L6237) writes a CONNECTION_CLOSE to the connection, but doesn't immediately set the connection's drain timer. This means that applications can write data to the connection which should effectively never be processed, by virtue of it being preceded by a CONNECTION_CLOSE.

We should just immediately set the draining timer to prevent sending extra data over the wire