Handle URIs longer than 255 characters

Store the md5 hash of page URIs as well as the original URL as a text field.  Only index the uri_hash field, and query against that instead of the URI.

sites.base_url is still a varchar(255) length.  This is because we do a lot more with that field in terms of querying and comparing.  I have yet to see a base_url over 138 in length.