[#86] netmap: Add netmap candidate list getter

With `NetmapCandidate` storage nodes can monitor
if they are expected to be in the netmap at next
epoch or not. This is also important for monitoring
when some nodes does not bootstrap immediately.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

netmap/config.yml

@@ -1,5 +1,5 @@
 name: "NeoFS Netmap"
-safemethods: ["innerRingList", "epoch", "netmap", "snapshot", "snapshotByEpoch", "config", "listConfig", "version"]
+safemethods: ["innerRingList", "epoch", "netmap", "netmapCandidates", "snapshot", "snapshotByEpoch", "config", "listConfig", "version"]
 events:
   - name: AddPeer
     parameters:

netmap/netmap_contract.go

@@ -351,6 +351,13 @@ func Netmap() []storageNode {
 	return getSnapshot(ctx, snapshot0Key)
 }
 
+// NetmapCandidates returns all node candidates for next epoch and their
+// status codes.
+func NetmapCandidates() []netmapNode {
+	ctx := storage.GetReadOnlyContext()
+	return getNetmapNodes(ctx)
+}
+
 func Snapshot(diff int) []storageNode {
 	var key string