HTTP2::Error::ProtocolError request has been resend to an out-of-session connection

Took your advice from #364 (closed) and we're now running 41661a05 and seeing this new error:

HTTP2::Error::ProtocolError
request has been resend to an out-of-session connection, and this should never happen!!! Please report this error! (state:open, parser?:true, bytes in write buffer?:false, cloned?:false, sibling?:false, coalesced?:) (HTTP2::Error::ProtocolError)

HTTPX::Error
request has been resend to an out-of-session connection, and this should never happen!!! Please report this error! (state:open, parser?:true, bytes in write buffer?:false, cloned?:false, sibling?:false, coalesced?:) (HTTPX::Error)

Alongside that, we also had a smaller number of instances of a timeout error, which I'm reporting in case it is the root error:

HTTPX::ReadTimeoutError
Timed out after 60 seconds (HTTPX::ReadTimeoutError)