open and close methods for handlers

[Tobias Rübner <de...@tobr.eu>]

The handler performs the processing of the data.
For many handlers you need to open and close the connection.
These are very exprensive operations and they should be performed as
least as possible.

For example the solr handler should commit its data only at the end of
the crawl.
Currently I implement it to the droid finish method, but checking for
the used handler, which is not very smart.

Maybe we can add this to the Handler API.

What do you think?
Tobias