this is necessary to avoid wearing clown shoes when the node stays
offline for an extended period of time (more than 1 finality).
Basically it gets quite slow if we do the full 2 finality walk, so we
try to avoid it unless necessary.
The conditions under which a full walk is necessary is if there is a
sync gap (most likely because the node was offline) during which the
tracking of writes is inaccurate because we have not yet delivered the
HeadChange notification. In this case, it is possible to have
actually hot blocks to be tracked before the boundary and fail to mark
them accordingly. So when we detect a sync gap, we do the full walk;
if there is no sync gap, we can just use the much faster boundary
epoch walk.
Calling, e.g., `lotus chain sethead` on an ancestor tipset won't apply
any new blocks, it'll just revert a bunch. This will lead to HeadChange
calls with no new blocks to apply.
fixes#6125
