Optimize size of did data structure

[noandrea]

The DID documents persisted are not optimized for size, but there are some
actions that can be done to optimize the storage for size:

• do not store the did prefix in the document and index:
  the prefix can be derived by the chain name and the did method name

• do not repeat the fully qualified did in the did document sections
  the fully qualified did is used eg. in verification methods, verification relationships and so on, they could be removed and substituted on the fly, incurring in a processing cost but saving space

[noandrea]

rejected