squid.conf cleanup: Modify several squid.conf defaults

squid.conf cleanup: Modify several squid.conf defaults

Following the cleanup of squid.conf to minimal config modifies the
remaining defaults to make their explicit configuration unnecessary.

    icp_port was made a 0 default (for safety?),
        but the port config line left uncommented. fixed that.
        (most won't need it, those who do need to configure it anyway)

    icp_access lines to allow local network now commented out,
        background default 'deny all' untouched.
        (ditto on above reason)

    miss_access default moved from explicit configured, to
        background default. Implicit absent default was documented
        to be same as explicit config default anyway.

    access_log config moved to a background default + documented.
        rather than explicit config only.

    cache_store_log moved to default none + commented out.
        We've been recommending that for a while now anyway.

    request_header_max_size    boosted to 64KB from 20KB.
        HTTP/1.1 needs big headers. I think that should be okay?

    reply_header_max_size    boosted to 64KB from 20KB.
        HTTP/1.1 needs big headers. I think that should be okay?

    cache_dir defaults to no disk cache, memory only cache.

    maximum_object_size_in_memory - boosted to 512KB.
        Update to at least 64KB was needed anyway to match modern web
        traffic. Picked 512KB to maximize HIT with new default cache.

    cache_mem boosted to 256 MB for caching at least 500 objects.

TODO Options remaining to consider for removal:
    hierarchy_stoplist
    coredump_dir

TODO all the default values probably still need to be checked.