This is a simple utility/wrapper class, that holds the field
values for recently indexed documents until the NRT reader has
refreshed, and exposes a "get" API to get the last indexed value per
For example one could use this to look up the "version" field for a
given id, even when that id was just indexed and not yet visible in
the NRT reader.
The implementation is fairly simple: it just watches the gen coming
out of NRTManager and updates/prunes accordingly.
The class is abstract: you must subclass it and impl the lookupFromSearcher