Handle several databases in parallel with dbi plugin

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.

The patch is pretty minimal. I think there is no additional lock needed even if several threads are now sharing the global properties. To the best of my understanding, the configuration initialization is always done in one thread only and only the read callbacks can be run in several distinct threads. Because the global information is accessed read-only in this case, no problem should occur.