IF: Update finalizer safety information and rules for how a finalizer signs

[greg7mdp]

resolves #2070.

This PR implements support for managing a set of stateful finalizers (the local finalizers that this node is running).

It is important that the safety information for each finalizer (last vote and proposal lock) is preserved across nodeos invocations, since incorrect voting can lead to finality violations.

The safety information for all finalizers is saved in a file named safety.dat, which is by default located in {data_dir}/finalizers. This file preserves the safety information for not only the currently running finalizers, but for all thefinalizers who ever ran on this node with the same specified safety file location.

The currently running finalizers are always the first ones stored in the file, followed by all the inactive ones. After each vote, only the information for the currently running finalizers is written to the file, which is then flushed. This file update occurs before the vote is propagated.

The way the finalizer safety information is initialized (if not found in the persistent safety.dat file) is important, and described here. The initial configuration must be such that it will prevent safety violations, while still allowing liveness to eventually happen, so the finalizer can vote.

This specification may yet evolve, as the document is not 100% completed.

This PR also includes some other misceallenous changes:

• update fc/crypto bls primitives to take std::span instead of std::vector, so we don't have to do unnecessary copies.
• correct support for weak digest (which is the strong digest with the 4 characters "WEAK" appended at the end). since we couldn't use digest_type, a custom type weak_digest_t was defined.
• extract a couple of utility functions from tester.cpp into a new file bls_utils.hpp, so that they can be reused in boost tests.

[linh2931]

I thought this name again. It seems my_fianlizers is more accurate, as we are talking about the finalizers on the local node. In configuration instructions to BPs, local finalizer sounds more natural than my finalizers. my implies something at implementation level like a class or a file. @heifner @arhag ?

[greg7mdp]

This is a class name at the implementation level.

[linh2931]

I believe in Boost unit tests, when safety file is the default, those simulated weak vote tests should have passed. Should come back to revisit those later.