[MEDIUM] indicate when we don't care about read timeout

[MEDIUM] indicate when we don't care about read timeout

Sometimes we don't care about a read timeout, for instance, from the
client when waiting for the server, but we still want the client to
be able to read.

Till now it was done by articially forcing the read timeout to ETERNITY.
But this will cause trouble when we want the low level stream sock to
communicate without waking the session up. So we add a BF_READ_NOEXP
flag to indicate that when the read timeout is to be set, it might
have to be set to ETERNITY.

Since BF_READ_ENA was not used, we replaced this flag.