Details
Minor Description
Set the context as IOContext.READ / IOContext.READONCE where applicable
Motivation:
Custom PostingsFormat may want to check the context on SegmentReadState and branch differently, but for this to work properly the context has to be specified correctly up the stack.
For example, DirectPostingsFormat only loads postings into memory if the context != MERGE. However a better condition would be context == Context.READ && !context.readOnce.