remove O(n^2) slow slow indexing defaults in DataImportHandler

By default, dataimporthandler optimizes the entire index when it commits.

This is bad for performance, because it means by default its doing a very
heavy index-wide operation even for an incremental update... essentially
O(n^2) indexing.

All that is needed is to set optimize=false by default. If someone wants
to optimize, they can either set optimize=true or explicitly optimize themselves.