conn: Stop writing when our write bandwidth limist is exhausted

Commit 488e2b00 introduced an issue, most
likely introduced by a bad copy paste, that made us stop reading on the
connection if our write bandwidth limit was reached.

The problem is that because "read_blocked_on_bw" was never set, the connection
was never reenabled for reading.

This is most likely the cause of #27813 where bytes were accumulating in the
kernel TCP bufers because tor was not doing reads. Only relays with
RelayBandwidthRate would suffer from this but affecting all relays connecting
to them. And using that tor option is recommended and best practice so many
many relays have it enabled.

Fixes #28089.