Lucene uses a non-trivial baseline bytes of heap for each indexed
field, and I know it's abusive for an app to create 100K indexed
fields but I still think we can and should make some effort to reduce
heap usage per unique field?
E.g. in block tree we store 3 BytesRefs per field, when 3 bytes
|Fix readStringStringMap api||Closed||Unassigned|
|re-use immutable maps across fieldinfos when they are the same||Closed||Unassigned|
|improve perf and memory of FieldInfos.fieldInfo(int)||Closed|