- make the entry cache preload backend implementation specific
  moving EntryCachePreloader to JEB package and reworking it
  for JEB specific preload.

- scale JEB entry cache preload by unloading decoding/encoding
  activities onto separate workers.

- use blocking queue to reduce in heap memory footprint, read
  ahead only when necessary.

- preload entry cache with backends in read only state, holding
  the server startup until the entry cache preload is complete.