core: add validUntilBlock field to transaction

1. closes #841

2. Commented out test cases where binary transaction are used.
These test cases marked with `TODO NEO3.0: Update binary` and need to be
updated.

3. Updated other tests.

4. Added cache to calculateValidUntilBlock() RPC-client method.

pkg/rpc/client/client.go

@@ -24,6 +24,8 @@ const (
 	defaultDialTimeout    = 4 * time.Second
 	defaultRequestTimeout = 4 * time.Second
 	defaultClientVersion  = "2.0"
+	// number of blocks after which cache is expired
+	cacheTimeout = 100
 )
 
 // Client represents the middleman for executing JSON RPC calls
@@ -40,6 +42,7 @@ type Client struct {
 	wif        *keys.WIF
 	balancerMu *sync.Mutex
 	balancer   request.BalanceGetter
+	cache      cache
 }
 
 // Options defines options for the RPC client.
@@ -57,6 +60,18 @@ type Options struct {
 	Version string
 }
 
+// cache stores cache values for the RPC client methods
+type cache struct {
+	calculateValidUntilBlock calculateValidUntilBlockCache
+}
+
+// calculateValidUntilBlockCache stores cached number of validators and
+// cache expiration value in blocks
+type calculateValidUntilBlockCache struct {
+	validatorsCount uint32
+	expiresAt       uint32
+}
+
 // New returns a new Client ready to use.
 func New(ctx context.Context, endpoint string, opts Options) (*Client, error) {
 	url, err := url.Parse(endpoint)