Squid-to-client write_timeout triggers client_lifetime timeout (#747)

Squid-to-client write_timeout triggers client_lifetime timeout (#747)

Since commit 5ef5e5c, a socket write timeout triggers two things:
* reporting of a write error to the socket writer (as designed/expected)
* reporting of a socket read timeout to the socket reader (unexpected).

The exact outcome probably depends on the transaction state, but one
known manifestation of this bug is the following level-1 message in
cache.log, combined with an access.log record showing a
much-shorter-than-client_lifetime transaction response time.

    WARNING: Closing client connection due to lifetime timeout