AUFS: Raise I/O queue congestion limits

AUFS: Raise I/O queue congestion limits

... from 8 to 8196 before initial congestion message appears.

Modern networks can be quite busy and even amateur installations have a
much higher I/O throughput than Squid was originally designed for. This
often results in a series of "Queue congestion" warnings appearing on
startup before Squid learns what the local environment requires.

The new limit helps to cater for this and reduce teh frequency of
unnecessary warnings. They may still occur, so debug output is also
updated to show what the queue length has grown to with each warning.

Also updating the congestion counter from 32-bit to 64-bit unsigned
since the new limit already consumes half the available growth bits in
32-bit integer.

NP: this update was triggered by reports from admin with proxies needing
    to expand AIO queues to over 4K entries on startup.