[PATCH] Modify icap_service_failure_limit option to forget old ICAP errors

This is the modified "icap_oldest_service_failure" patch posted by Alex
some days before.

This patch instead to add the new option "icap_oldest_service_failure",
modifies the icap_service_failure_limit options as requested by the
squid developers.
The new syntax of icap_service_failure_limit options is:
    icap_service_failure_limit limit [in memory-depth time-units]
Example:
     icap_service_failure_limit 10 in 5 seconds

A detailed description included in the patch.
Author is Alex Rousskov

Regards,
     Christos