Mithril signer should have a unique protocol initializer for an epoch

[jpraynaud]

Why

The Mithril signer is not reusing the same protocol initializer for a given epoch. If it is restarted, it will create a new one.
This behavior can become problematic in a decentralized setup of the signer registration and be interpreted as an equivocation attack.

What

Reuse the same protocol initializer in a given epoch for a signer.

How

• Reuse the protocol initializer of the same epoch to do the signer registration