Unbind event listeners on close (#1824)

for uncaught exceptions and rejections. Closes a small memory leak observed in Issue #1706.

Author: wbt

lib/winston/logger.js

@@ -402,6 +402,8 @@ class Logger extends Transform {
    * @returns {Logger} - TODO: add return description.
    */
   close() {
+    this.exceptions.unhandle();
+    this.rejections.unhandle();
     this.clear();
     this.emit('close');
     return this;