Make helper queue size configurable, with consistent defaults and better overflow...

Make helper queue size configurable, with consistent defaults and better overflow handling.

This patch adds a queue-size=N option to helpers configuration. This
option allows users to configure the maximum number of queued requests
to busy helpers. We also adjusted the default queue size limits to be
more consistent across all helpers and made Squid more robust on some
queue overflows:

- external_acl helpers
    Make the maximum queue size configurable via queue-size.
    Default to 2*maximum-number-of-children.
    If the queue overflows, then the ACL returns ACCESS_DUNNO.

    Unpatched code uses the number of running children as the maximum
    queue size. If the queue is overloaded, then the ACL returns ACCESS_DUNNO.

-redirector/storeID helpers
    Make the maximum queue size configurable via queue-size.
    Default to 2*maximum-number-of-children.
    If the queue overflows and redirector_bypass configuration option
    is set, then redirector is bypassed. Otherwise, if overloading
    persists for more than 3 minutes squid quits with a FATAL message.
    If the redirector_bypass/storeID_bypass is set then the default queue_size
    is set to 0 for backward compatibility.

    Unpatched code uses 2*number-of-running-children as the maximum queue size.
    If the redirector_bypass/storeID_bypass is set then helper bypassed if all
    of the children are busy.
    If the queue is overloaded and redirector_bypass/storeID_bypass is not set
    then squid quits with a FATAL message.

- ssl_crtd/ssl_crtd_validator helpers.
    Make the maximum queue size configurable via queue-size.
    Default to 2*maximum-number-of-children.
    If the queue overflows, then helpers are bypassed. If overloading persists
    for more than 3 minutes squid quits with a FATAL message.

    The default size limit and overflow behavior has not changed.

- Authentication helpers
    Make the maximum queue size configurable via queue-size.
    Default to 2*maximum-number-of-children.
    If the queue overflows and overloading persists for more than 3 minutes,
    then squid quits with a FATAL message.

    The default size limit and overflow behavior has not changed.

This is a Measurement Factory project