WIP: Add metadata to store record (#1604)

* Add metadata to store record

* Add metadata to cockroach store

* add metadata to store service implementation

* fix breaking cache test

* Test/fix cockroach metadata usage

* fix store memory metadata bug

store/cockroach/metadata.go

@@ -0,0 +1,45 @@
+package cockroach
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+)
+
+// https://github.com/upper/db/blob/master/postgresql/custom_types.go#L43
+type Metadata map[string]interface{}
+
+// Scan satisfies the sql.Scanner interface.
+func (m *Metadata) Scan(src interface{}) error {
+	source, ok := src.([]byte)
+	if !ok {
+		return errors.New("Type assertion .([]byte) failed.")
+	}
+
+	var i interface{}
+	err := json.Unmarshal(source, &i)
+	if err != nil {
+		return err
+	}
+
+	*m, ok = i.(map[string]interface{})
+	if !ok {
+		return errors.New("Type assertion .(map[string]interface{}) failed.")
+	}
+
+	return nil
+}
+
+// Value satisfies the driver.Valuer interface.
+func (m Metadata) Value() (driver.Value, error) {
+	j, err := json.Marshal(m)
+	return j, err
+}
+
+func toMetadata(m *Metadata) map[string]interface{} {
+	md := make(map[string]interface{})
+	for k, v := range *m {
+		md[k] = v
+	}
+	return md
+}