Support multiple crypto algorithms per engine

[andreashappe]

Currently an archistar engine is started with one configured crypto-algorithm (i.e. ShamirPSS). All fragments stored on the storage server have metadata describing the used secret sharing mechanism.

So as a frist step it should be easy to use one algorithm for outgoing (write) data and instantiate different CryptoEngines for each used crypto-algorithm for incoming data.

A long-term solution might make choosing the outgoing (read) crypto-algorithm policy-controlled, ie. use different algorithms for different data.