Prevent spurious error when muxer closes

[themighty1]

Currently at the end of the finalization the Verifier sends two messages:

• a SignedSessionHeader
• a signal for the Prover to shut down the muxer

The prover polls the associated futures here:

tlsn/tlsn/tlsn-prover/src/tls/mod.rs

Lines 315 to 319 in ce80e75

	let (notary_encoder_seed, SignedSessionHeader { header, signature }) = futures::select_biased! {
	res = notarize_fut => res?,
	_ = ot_fut => return Err(OTShutdownError)?,
	_ = mux_fut => return Err(std::io::Error::from(std::io::ErrorKind::UnexpectedEof))?,
	};

It is possible that before notarize_fut finished processing SignedSessionHeader, the mux_fut will complete causing the prover to return an error.

We should find a way to prevent such a spurious error.