Memory leak fix: do not wait for process.nextTick to clear pending …

…callbacks (#2057)

* Memory leak fix: do not wait process.nextTick 

Addresses the memory leak identified here:

#1871 (comment)

* resolve linting failure

lib/winston/logger.js

@@ -312,6 +312,7 @@ class Logger extends Transform {
     try {
       this.push(this.format.transform(info, this.format.options));
     } finally {
+      this._writableState.sync = false;
       // eslint-disable-next-line callback-return
       callback();
     }