Add custom Store ID code support

Add custom Store ID code support

This is a port of the StoreURL feature from Squid-2.7 rewritten to avoid
the unfortunate link that 2.7 feature had with URL-rewriting.

The feature uses a helper to retrieve custom ID values for use in the
cache storage ID key. The default ID used is the requested URL.

Differences since 2.7:
* storeurl_* directive names are now called store_id_*

* The helper response now expects 'OK store-id="..."\n'
  However the squid-2.7 response syntax is still accepted.

* the ID value presented need not be a URL. Although URL syntax
  is recommended to simplify store log interpretation and
  refresh pattern matching.

* refresh_pattern applies to the store ID, not the request URL.

TODO: support store ID lookups on ICP and HTCP queries.