Conversion of existing second signature and multi signature accounts

[shuse2]

Description

After implementing #4839, all the testnet and mainnet second signature and multi signature accounts need to be migrated to the new multisignature format.

Motivation

New multisignature is migratable from the old account schema.

Acceptance Criteria

• Conversion rule should follow Make multi-signature accounts more flexible, prevent spamming, and prevent signature mutability
• Test second signature are migrated properly
• Test multi signature account are migrated properly
• Test multi signature with second signature accounts are migrates properly

Additional Information

• Add migration to remove the second signature and second publicKey from mem_accounts

[shuse2]

Closing this because this will handle by migration scripts