Rename chainsync to legacychainsync

[ezdac]

1/4 - Rename chainsync to legacychainsync

This is part of a series of 4 Pull Requests that all aim to refactor our old
chainsync mechanism. The reason to split this up is to provide more digestible
and grouped parts of the refactoring effort. I am aware this is a big change,
but especially for refactoring an integral part of the system, and designing
it at the same time, at least for me it is impossible to provide smaller
increments of work that don't leave the code in an uncompilable,
non-functional state.

The chainsync mechanism will be refactored in the following PRs.
It will temporarily be still in use in the follow-up PRs of the refactoring, and the
optimism keyper package will still use the legacychainsync. This is because integrating the
new chainsync mechanism in the optimism keyper was currently no priority.