There is a need to test each `Client` operation. In previous
implementation `Client` was based on real socket connection. This didn't
allow to test the `Client` without OS resources. In order to write
convenient and useful unit tests we need to slightly refactor the code.
Introduce `neoFSAPIServer` interface as a provider of `Client` type's
abstraction from the exact NeoFS API server. Add `netMapSnapshot` method
for initial implementation. Define core interface provider used in real
code. Set `coreServer` as an underlying `neoFSAPIServer` in
`Client.Dial`. Cover `Client.NetMapSnapshot` method with unit tests
using the opportunity to override the server.
From now client library can be tested not only with real physical
listeners but with imitations.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
