Move FundingScope-specific validation to ChannelContext

Previous commits refactored validation checks in FundedChannel to work
on a specific FundingScope. However, keeping these helpers in
FundedChannel doesn't prevent them from using self.funding inadvertently
instead of the passed in FundingScope. Move these helpers to
ChannelContext to avoid this problem, as we done with similar helpers.

Author: jkczyz