[avalanche] Extract a template class from BlockUpdate

Summary:
So we can reuse it for conflicting proofs as well. Note that this removes the space optimization (using the 2 LSB from the pointer to store the status) because conflicting proofs will use a std::shared_ptr<Proof> which can't be cast to uintptr_t.

There is no change in behavior.

Ref T1854.

Depends on D10249.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10250