Make `tokens`, `authmate` and `layer` packages to depend from locally
defined `NeoFS` interface of the virtual connection to NeoFS network.
Create internal `neofs` package and implement these interfaces through
`pool.Pool` there. Implement mediators between `NeoFS` interfaces and
`neofs.NeoFS` implementation.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
