[nspcc-dev#269] netmap: Return same type from reading methods

There is a need to return similar structure of information about the
storage nodes from the contract storage readers. In previous
implementation some methods didn't return node state which can differ
with the one encoded in the node's BLOB.

Define `Node` structure of the information about the storage nodes
recorded in the contract storage. Return `[]Node` from all related
methods.

Also improve docs of touched contract methods.

Signed-off-by: Leonard Lyubich <[email protected]>