Optionally disable inserts on boost_cache table

This is more of a feature request rather then a bug report.

Once the file is created, it will never be overwritten; editing a node will not remove the old file. boost_cache_expire() uses the database instead of glob when flushing a node on edit. Your current setup is on the edge if this is slowing down you DB.

Anyway here's a patch... Key part is inside boost_cache_set() disabling

    boost_db_prep($filename, $extension, BOOST_TIME + $expire);
    boost_cache_set_node_relationships($GLOBALS['_boost_relationships']);

If I get multiple requests for this (10+) or get paid, or if a better patch comes out I will commit it & support this feature request.

This might be of interest
#610198: Allow flush button to del a file if not in database

This isn't that painful to do actually... so here it is.

committed