dbi plugin: fix a warning mentioning snmp plugin
dbi plugin: register one read callback for each database block

By using a distinct read callback for each database block, collectd
will be able to query several databases in parallel. This is useful if
the plugin should handle many queries. This lets the user split them in
chunks and speed up the retrieval.
Merge pull request #453 from vincentbernat/feature/parallel-dbi

Handle several databases in parallel with dbi plugin