MEDIUM: mux-h2: Introduce flags to deal with connection read/write errors

MEDIUM: mux-h2: Introduce flags to deal with connection read/write errors

Similarly to the H1 multiplexer, H2_CF_ERR_PENDING is now used to report an
error when we try to send data and H2_CF_ERROR to report an error when we
try to read data. In other funcions, we rely on these flags instead of
connection ones. Only H2_CF_ERROR is considered as a final error.
H2_CF_ERR_PENDING does not block receive attempt.

In addition, we rely on H2_CF_RCVD_SHUT flag to test if a read0 was received
or not.