Handle collectd counter/derive conflict

If a collectd metric is received with a value of type counter 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. Also, it's common
for different versions/installations of collectd in 'clients'
to have a bit different definitions for the same metrics
(counter/derive conflict). This commit makes bucky's collectd
server handle such cases gracefully by assuming the server's
types.db is correct in case of conflict.
Add conf option to ignore counter/derive conflicts

in collectd, False by default.
Add test for collectd_counter_eq_derive
Merge branch 'collectd_counter_eq_derive' of github.com:dimrozakis/bucky into dimrozakis-collectd_counter_eq_derive
Merge branch 'dimrozakis-collectd_counter_eq_derive'