[PATCH] add support for write I/O timeout / squid-3.1 version

This is the squid-3.1 version of the write I/O timeout patch.

Description:
 The write I/O timeout should trigger if Squid has data to write but
the connection is not ready to accept more data for the specified time.
If the write times out, the Comm caller's write handler is called with
an ETIMEDOUT COMM_ERROR error.

Comm may process a single write request in several chunks, without
caller's knowledge. The waiting time is reset internally by Comm after
each chunk is written.

Default timeout value is 15 minutes.

-
  Christos