message: Update Verifiable Storage and Tails design.
diff: # Goals
  -* Provide default implementation on indy-sdk level for tails routine
  +* Provide default implementation on indy-sdk level for Tails routine
  * Need to be able to extend and customize tails handler in applications
  -* * For example allow client to not downloads whole large tails
  + * For example allow client to not downloads whole large tails
  # Tails
  -Expected size is about 1/2 GB per Issuer revocation registry.
  -"As is" tails is static (once generated) array of BigIntegers
  +It's an ordered sequence of elements used for non-revocation proof. "As is" tails is static (once generated) array of BigIntegers
  +* may require quite huge amount of data (up to 1/2GB per Issuer revocation registry);
  +* are created and uploaded by Issuers;
  +* are required (so must be available for download) for both Provers and Verifiers;
  +* can be cached and can be downloaded only once;
  +* it's may not be enough just to store tails on Issuer side, as Issuer may be offline
  ## Required Tails Functionality
  * generate_tails
  * init_tails_storage
  -* deinit_tails_storage
  * get_tail
  ## Indy Crypto API for tails operate
  -TODO
  +* Some calls in Anoncreds API are require access to tails for read.
  + For these calls `fetch_tail(context, index)` should be passed.
  +* There is ability to generate Revocation Registry (including tails) by IndyCrypto.
  + The previous version API has `new_revocation_registry` method returning public and private part of the registry.
  + And whole tails are included into public part.
  + The new one version of API should return public part of revocation registry without Tails. And one more method should be added `generate_tails(rev_reg_private, from_idx, to_idx, *buf_ptr, buf_sz)`.
  + * `rev_reg_private` private part of `new_revocation_registry` result
  + * `from_idx..to_idx` - range of tails to generate (include `from`, exclude `to`)
  + * `buf_ptr` is allocated by caller buffer to store generated tails and `buf_sz` is size in bytes of this buffer
  # Verifiable Storage (VS)
  Indy SDK can implement some generic storage with verify ability.
  