Refactors and (significantly) improves performance of the file exporter.

• Adds use of System.Threading.Channels to improve clarity and shutdown handling.
• Now keeps open the current trace file until full or disposed.
• Only checks for deleting old files when a new file is created.
• Adds a handler for OnForceFlush to try to flush messages when needed.
• Improved shutdown/dispose reliability to flush messages.

Benefits

• orders of magnitude improvement in performance
• huge improvement in concurrent use in the same folder