Indexer batched unindexing

[Enet4]

The current definition for unindex can take a single URI. As most implementations of this imply one index write for each removal, this constitutes a performance bottleneck.

If an overload was provided for unindexing a list of items, implementations could more easily attain better performance by writing all changes to the index in a batch, rather than flushing each operation one by one.

public ? unindex(List<URI> path) throws ?;

[bastiao]

It is a nice idea coming from a real user story. :) Definitely, we should go for it.