[PKI] Organaze storage-related logic in x/pki app module better

[spivachuk]

Now in x/pki app module Keeper handles addition of unique certificate keys separately from addition of certificates. However, unique certificate keys are registered at the same moments when certificates are added / proposed. Actually unique certificate keys are just additional indexes of certificates. So it makes sense to put addition / proposal of a certificate and addition of the unique certificate key into one operation (addition / proposal of a certificate correspondingly).

Also it makes sense to move logic currently located in addChildCertificateEntry and removeChildCertificateEntry methods of Handler to Keeper.

[ashcherbakov]

We need to define the rule for the boundaries between a Keeper and a Handler (not for PKI module only).
I think the rule can be the following:

• A Handler's method should consist of two parts:
  • validation (can call Keeper if needed)
  • call one (and only one) Keeper's method to store the entity in the State
• A Keeper's method for storing entities must make it in an "atomic" way so that it
  • stores the main entity in the state
  • create or update additional entities or indices

[ashcherbakov]

#157
Keeprs contain methods to work with one key only.
Handlers (Msg Servers) contain business logic to potentially work with multiple keys.