* 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`
* Remove `--sync-mode` option from nimbus config
why:
Currently there is only one sync mode available.
* Rename `flare` -> `beacon`, but not base module folder and nim source
why:
The name `flare` was used do designate an alternative `beacon` mode that.
Leaving the base folder and source as-is for a moment, makes it easier
to read change diffs.
* Rename `flare` base module folder and nim source: `flare` -> `beacon`
