This PR renames the existing `validator_duties` to `beacon_validators`
and in doing so, names validators running inside the beacon node process
"beacon validators" while those running the VC can be referred to as
"client validators" to disambiguate the two.
The existing `validator_duties` instead takes on a new responsibility:
as a home for logic shared between beacon and client validators - ie
code that provides consistency in implementation and behavior between
the two modes of operation.
Not only does this simplify reasoning about where to put code -it also
reduces the number of dependencies the validator client has from ~5000
to ~3000 modules (!) according to `nim genDepend` significantly reducing
compile times.
Jacek Sieka