Spinoff from this thread:
http://markmail.org/message/bojgqfgyxkkv4fyb
I think it makes sense ask an IndexReader for the commit point it has
open. This enables the use case described in the above thread, which
is to create a deletion policy that is able to query all open readers
for what commit points they are using, and prevent deletion of them.
I also deprecated the original interface IndexCommitPoint, and
switched to abstract base class IndexCommit instead, and added
equals() method to IndexCommit. I also added "warning this API is
still experimental" to the javadocs too.