Minor refactoring.
Only load event indexes when a query needs them.
Reprocess segment as new hits arrive.
Beef up range_map.
Support backwards seeking of segment reader.

As a consequence, we renamed skip_to to seek to reflect the bidirectionality.
Implement backward searching in bitvector.

The member functions find_last() and find_prev() represent the dual to
find_first() and find_next().