Router Additions

[mookums]

This PR iterates on the current Router by bringing in both QOL features and necessary security hardening.

• Router now provides a serve_not_found route where you can provide a handler for deciding what happens when a Not Found route is triggered. The basic example is updated to highlight use of it.
• Router now ensures that any queries to the serve_fs_dir path rely within the path, preventing path traversal attacks that may have been possible before.
• Router now will attach an ETag to any files served through serve_fs_dir, allowing for rudimentary caching using the size and last modified time. It will automatically return 304 if the browser/client requests a matching file.
• Serving from the filesystem no longer requires the entire file to be copied into memory, reducing both the memory footprint of serve_fs_dir and also allowing for files that are larger than RAM to be served (eg. 100 GB file on 16 GB of RAM). This is thanks to Tardy.
• Fixes a bug within the SSE implementation that may have led to connections entering a recv state if the data that was being sent to the client got fragmented by the kernel.