implement files cache for --no-cache-sync

[ThomasWaldmann]

in borg 1.1, we'll have experimental borg create --no-cache-sync, but there is currently no files cache with it.

running borg create over a bigger amount of files (count as well as size) takes ages as it will read all files to chunk them.

[ThomasWaldmann]

hmm, if we do that, guess we could use chunks = [(id, size), ...] in the files cache to work around that we do not have the size information in the ad hoc chunks index when not having produced the chunk in the current session (also, we can not easily get the size information from anywhere else, like from the repo).