On-disk storage of (immutable) certificates

[amesgen]

We need to store certificates boosting immutable blocks indefinitely in order to serve them to syncing peers.

The goal of this ticket is to design the corresponding storage component. This component must efficiently support the operations required by the mechanism we use to diffuse certificates to syncing peers (see #3).

Important to keep in mind: It can be the case that a block is boosted by multiple certificates.

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Two ideas:

• A new, independent component of the ChainDB. Implementation could be backed by something similar to the existing ImmutableDB, or potentially by the lsm-tree lbrary.
• We could enrich the ImmutableDB to store certificates next to their corresponding blocks. One neat thing about this approach is that certain certificates (before cooldown periods) will be stored in block bodies either way.