[PATCH] add support for write I/O timeout

This patch add support for write timeouts.
The patch also exist for squid3.1 branch
The development sponsored by the Measurement Factory

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.

The implementation requires no changes in Comm callers but it adds write
timeouts to all connections, including the connections that have
context-specific write timeouts. I think that is fine.

-
  Christos