Replace storage::BlockId with common::BlockId

[Mirko-von-Leipzig]

Should only be done after #2038 and #1738.

This will be quite a painful refactoring, but should result in less code branches within RPC methods.

Be aware that some methods might still require special handling of the pending branch. e.g. pending block headers are a subset of the full block header.

Once completed, the transaction::pending_data method should be made private which will ensure that the work is completed correctly.