refactor(beacon-chain): potential race to write callbacks

[mattevans]

tldr; running go test with -race is picking-up that this insn't thread-safe.

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I'm writing some unrelated e2e tests for contributoor and was getting some unusual failures stemming from this pkg when running tests with -race.

This is not a problem for production right now, given how we typically use this pkg during initialization

Initially I thought this was related to our multi-beacon-node changes in contributoor:

• Each MetadataService creates its own ethwallclock instance
• Each instance has its own independent callback slices
• There's no sharing of callbacks between instances
• The race detector was catching races within individual instances, not between them. Isn't smart enough to tell the difference.

If you run go test -v -race ./... on master of this repo, you'll see the race's/failures in question.

I'd love to keep using -race across contributoor and other pkgs, so hence this PR.