fix(core-transaction-pool): disallow multiple registrations for same delegate

[faustbrian]

Proposed changes

Only allow a single instance of a delegate registration in the pool per address.
Closes: https://github.com/ArkEcosystem/security-vulnerabilities/blob/master/core/core-sv-012.md

Types of changes

• Bugfix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes

[ghost]

@air1one @supaiku0 - please review this in the next few days. Be sure to explicitly select labels so I know what's going on.

If no reviewer appears after a week, a reminder will be sent out.