WebSession: allow longer session object data

• Enlarges session_object in the session table to be longblob in order to be able to store more search results hits there. This is necessary especially for sites with many collections, otherwise session truncation might have occurred.

• At the same time, diminishes CFG_WEBSEARCH_PREV_NEXT_HIT_LIMIT to 1000. This means that for queries giving more than 1000 search results the system will not show next/previous links on the detailed record pages. (This is a safer default and users are not likely to browse through detailed record pages for more hits anyway.)