Revert "Fix external command pipe: Too many open files"

This reverts commit 1a3f7d774c2dce7750103913b2864c3c588b386f.
Revert "ExternalCommandListener: Use threads per client connection"

This reverts commit accd83693ce699be5538a60facf1d11b5ec3ac57
Use threads for external commands

refs #6589
fixes #6942