Update README about loading performance. Update flag name to stw_ram_mb, which explains it's purpose better. Time to update docker image

Switch dirtylist with dirtymap, because list causes a merge explosion, when the same posting list gets multiple updates.

Clean up periodic cleaning and merging of lists. Now we have a list of dirty posting lists, which can be merged periodically. This is a lot better than iterating over the entire hashmap, and finding dirty lists there. Also, allow for clog to be nil, to make loading fast.

Use cache for streaming entries instead of opening files for each posting list. Use iterator instead of channel while streaming entries.