Improve multi-user/multi-client concurrency

[andreashappe]

Currently all metadata (directory information) is stored within an shared encrypted index file upon the storage servers. On write-access the current version of the index file is overwrite. Last write wins.

Can this situation be improved without moving additional knowledge into the storage server? I do like the current situation that the storage server treats the index as normal binary blob -- but this might prevent good concurrency (which means safety and/or performance).