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

[smashah]

Addresses the memory leak identified here:

winstonjs/winston#1871 (comment)

[DABH]

Similar to the comment on the Winston PR -- let's make sure this change doesn't break Winston tests, and if all is good, then we can merge this in.

To test this specific change, you can temporarily modify package.json for your local copy of winston and specify the version of winston-transport as ../winston-transport (e.g. the relative file path to your local copy of the repo, instead of a version number). Hope that is helpful. Thanks again for your work on this!