Refactoring and detaching modMcf from modDbImpl

[AlexandraRoatis]

Description

• Removed unnecessary type variable from Serializer
• DataSourceArray can no longer be accessed outside the db module:
  • Knowledge of the sizeKey is removed from external tests.
  • Internal module tests check for correct storage of the size key in exceptional error cases that may cause it to miss from the database.
  • Removed the use of Flushable interface which will be removed once all implementations are updated.
• ObjectDataSource can no longer be accessed outside the db module:
  • Removed reliance on Flushable interface.
  • Made implementations visible only inside the package.
• Removed Flushable interface (superfluous design).
  • Flush functionality inside PendingBlockStore was not needed and therefore removed.
• Removed unused flushCopiesTo method from AionRepositoryCache
• Removed unused return type form createAccount method in RepositoryCache.
• Removed some implementation-specific functionality from ContractDetails:
  • Removed DetailsProvider from configuration (superfluous design)
  • ContractDetailsCacheImpl no longer pretends to implement functionality to fit the interface.
  • modMcf dependency on modDbImpl is no longer required.

Type of change

Insert x into the following checkboxes to confirm (eg. [x]):

• Bug fix.
• New feature.
• Enhancement.
• Unit test.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.