Add missing functions from db package.

[cabrador]

This PR adds missing functionality needed by Aida

1. BaseDB
   getBackend() - returns the low level level.DB backend
2. SubstateDB
   NewSubstateTaskPool() is now a struct func of the SubstateDB
GetLastBlock() - returns last block of given DB
   GetFirstSubstate() - returns the first existing substate inside given DB (first blk and tx)
   GetLastSubstate() - returns the last existing substate inside given DB (last blk and tx)
3. MakeDefault...DB - we can now create any database type from any database type since every interface implements the BaseDB interface.

[evgensheff]

Might need to add unit tests for this functionality in the future, so far everything looks good to me