Handle collectd gauge/derive inconsistency

If a collectd metric is received with a value of type gauge when
our types.db define it as derive, or vice versa, don't raise an
exception and assume the server's types.db is correct.

Types counter and derive are very similar and only differ in
case of a counter wrap around. Also, it's common for different
collectd 'clients' to have a bit different definitions for the
same metrics (gauge/derive). This commit makes bucky's collectd
server handle such cases gracefully.