MySQL has a packet limit of 1MB as a default
(http://dev.mysql.com/doc/refman/5.0/en/packet-too-large.html).
This interferes with the UPDATE queries required
to add file content caches. Since the query can't be terminated
correctly, the whole content will be discarded with a query error.
This change allows to truncate content prior to the UPDATE operation,
and defaults to 500 characters. This leaves some room for multibyte
characters as well as other parts of the SQL query.
