* Fix fringe condition clarifying how to handle an empty range
why:
The `interval_set` module would treat an undefined interval construct
`[2,1]` as`[2,2]` (the right bound being `max(2,1)`.)
* Use the `consensus head` rather than the `finalised` block as sync target
why:
The former is ahead of the `finalised` block.
* In ctx descriptor rename `final` field to `target`
* Update docu, rename `F` -> `T`
f937f57838