Make sure ICommitter.commit() is only called once

[essiembre]

This ticket originated from Norconex/committer-elasticsearch#3 (comment).

Because of AbstractBatchCommitter calling commit() for every batch, this eliminates knowing when commit was actually called by the framework using the committer (e.g. crawler).

Actual commit operations should be delegated to an inner method and commit() would be available to overriding for those interested to know when it was actually called externally.

Alternative to consider: shall we have the equivalent of a close() method? This would force everybody using it to close each committer which is an extra burden that may not be super useful if we make sure commit() is only called at the end. Yet, since we cannot guarantee implementers will only call commit() at the end, maybe a close() method is best? We could have the ICommitter extends Closeable as well to help force people call the close method.

[essiembre]

This issue has been moved here: Norconex/committer-core#11