Replace getCanonicalHead from core-db to ForkedChain
The meaning of `getCanonicalHead` of core-db now has been shifted to `getBaseHead`. The one provide canonical head now is `ForkedChain`.
Change core-db `getCanonicalHead` into `getBaseHead` to avoid confusion. And add `getCanonicalHead` or `getHead` to `ForkedChain`.

BASE  >  FINALIZED HEAD > CANONICAL HEAD

see https://notes.status.im/nimbus-merge-first-el?both=#Glossary

we Might as well add `getFinalizedHead` to FC