Implement a b/c layer that allows upgrading storages from previous file format

[albe]

Sometimes breaking the file format (for the storage/partitions and indexes) is inevitable as seen with #80. In those cases right now the old data is no longer readable. The solution then would be to install two versions of this library, make the old one read the data (events), pipe it to an instance of the new version and write them there. This comes at least at the loss of original metainformation (timestamps) and is cumbersome.

A solution would be to implement a readonly support for the last file format version. Then transforming the old storage to the new format could be done automatically with as many adjustments as possible to keep the actual data intact.