Try to move some more norms/docvalues merging out of SegmentMerger [LUCENE-5744] #6806
Description
Today, SegmentMerger opens up all the norms/docvalues and calls merge on the codec.
Can we improve this? In general these things are lazy-loaded, but it would be nice for example, in the case of norms or memory docvalues, to not actually load up things into their hashmap during merging (this would be just a few lines of code), or maybe in the future, even stream the data from disk if not loaded already.
This would help a lot with RAM spikes during merging, which can happen if someone has a lot of "latent" norms, e.g. fields they rarely search on.
Migrated from LUCENE-5744 by Robert Muir (@rmuir)