Issue Description:

On Linux only, When attempting to encode JSON using JSONEncoder, whilst setting its .outputFormatting property to .sortedKeys, a memory leak appears which.

When running many (e.g. 12) concurrent tasks quickly (every ~0.5s) can build up to a very large amount used memory (~4GB in a few minutes).

I'm assuming this is a bug in the Linux implementation of this feature (or a dependancy), as this doesn't occur when running on macOS from Xcode.

(Note this was discovered in a Swift on the server app (using Kitura) on Ubuntu in a docker container, both locally and on AWS)