Restructure the mapping and syncing of chunks

[rakshasa]

The current reference counted StorageBlock? thing should go, it's to ugly and does not support some of the features required.

Make a class that inherits from std::vector, the elements hold the time since the last time a piece was read/writen. At regular intervals msync some range of chunks, preferably as continious as possible. The user is responsible for telling us when it is done with a chunk.