lock forkchoice on calls to Ancestor

[potuz]

Lock forkchoice on calls to Ancestor().

Fixes #12161

The function Ancestor() currently exposed in forkchoice is only accessed when processing attestations currently from the sync package. The single entry point is the function ancestor from the blockchain package. It would be safe to lock within forkchoice. However, as discussed on slack, we are moving to a system in which

1. Forkchoice should be only visible from withing the blockchain package (and possibly the stategen package)
2. Blockchain package locks forkchoice on appropriate calls
3. External packages that want to call forkchoice to get information (specifically RPC package) should do so via the blockchain package, through calls in chain-info.go that expose forkchoice with the appropriate locks.

This PR does the minimal amount of work to fix #12161 while adhering to the above design: we lock on a blockchain method (which is now exposed and moved to the chain-info.go file) and set a new interface within it with this exposed method.

In future PRs we should move methods that are simple wrappers to forkchoice, from different interfaces, to this interface and do not expose Forkchoicer() which gives access to all of forkchoice to other packages. This is a larger project that will require time.

[prestonvanloon]

LGTM