Allow more BIP0022 fields in GetBlockTemplateCmd.

BIP0022 defines optional fields in a getblocktemplate request for long
polling and template tweaking.

In addition, for template tweaking, there are two fields, sigoplimit and
sizelimit, which are atypical in that they are allowed to be either
booleans or numeric.  This requires the fields to be represented as
interfaces which means any code making use of the struct will need to use
type assertions or a type switch.

This commit updates GetBlockTemplateCmd accordingly.

ok @jcvernaleo

jsoncmd.go

@@ -2173,6 +2173,21 @@ func (cmd *GetBlockHashCmd) UnmarshalJSON(b []byte) error {
 type TemplateRequest struct {
 	Mode         string   `json:"mode,omitempty"`
 	Capabilities []string `json:"capabilities,omitempty"`
+
+	// Optional long polling.
+	LongPollID string `json:"longpollid,omitempty"`
+
+	// Optional template tweaking.  SigOpLimit and SizeLimit can be int64
+	// or bool.
+	SigOpLimit interface{} `json:"sigoplimit,omitempty"`
+	SizeLimit  interface{} `json:"sizelimit,omitempty"`
+	MaxVersion uint32      `json:"maxversion,omitempty"`
+}
+
+// isFloatInt64 returns whether the passed float64 is a whole number that safely
+// fits into a 64-bit integer.
+func isFloatInt64(a float64) bool {
+	return a == float64(int64(a))
 }
 
 // GetBlockTemplateCmd is a type handling custom marshaling and
@@ -2194,6 +2209,38 @@ func NewGetBlockTemplateCmd(id interface{}, optArgs ...*TemplateRequest) (*GetBl
 			return nil, ErrTooManyOptArgs
 		}
 		request = optArgs[0]
+		switch val := request.SigOpLimit.(type) {
+		case nil:
+		case bool:
+		case int64:
+		case float64:
+			if !isFloatInt64(val) {
+				return nil, errors.New("the sigoplimit field " +
+					"must be unspecified, a boolean, or a " +
+					"64-bit integer")
+			}
+			request.SigOpLimit = int64(val)
+		default:
+			return nil, errors.New("the sigoplimit field " +
+				"must be unspecified, a boolean, or a 64-bit " +
+				"integer")
+		}
+		switch val := request.SizeLimit.(type) {
+		case nil:
+		case bool:
+		case int64:
+		case float64:
+			if !isFloatInt64(val) {
+				return nil, errors.New("the sizelimit field " +
+					"must be unspecified, a boolean, or a " +
+					"64-bit integer")
+			}
+			request.SizeLimit = int64(val)
+		default:
+			return nil, errors.New("the sizelimit field " +
+				"must be unspecified, a boolean, or a 64-bit " +
+				"integer")
+		}
 	}
 	return &GetBlockTemplateCmd{
 		id:      id,