Unify DBLayer QSM model and DB.MVar

[rvl]

Context

To reduce maintenance burden we could either delete the MVar DBLayer, or have it share pure model code with the state-machine tests. Since the MVar DB is a useful diagnostic tool, let's keep it for now. Also its model datatype works better than the one in StateMachine.hs.

Decision

Create a module Cardano.Wallet.DB.Model and move the data types from MVar to there, and modify them for rollback.

type Database wid s t k =  Map wid (WalletDatabase s t k)
type TxHistory t = Map (Hash "Tx") (Tx t, TxMeta)
data WalletDatabase s t xprv = WalletDatabase
    { metadata :: !WalletMetadata
    , xprv :: !(Maybe xprv)
    , txHistory :: !(TxHistory t)
    , checkpoints :: ![Wallet s t]
    }

Acceptance Criteria

1. MVar DBLayer and DBLayer QSM tests share a model and its functions.
2. Model functions are pure and don't use MVars, IO, etc.
3. The model supports checkpoints, but only a single checkpoint is used.

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Development Plan

• Check that key type can be a parameter of db model.
• Set up module for DB model.
• Change StateMachine.hs to use it.
• Change MVar DB to use it

PR

Number	Base
#660	master

QA

This is a refactor so the existing tests will cover it.

[piotr-iohk]

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