Nyzo 625: relay cache invalidation

Nyzo version 625 (commit on GitHub) adds automatic invalidation of cached files on the relay server based on file modification timestamps.

This version affects the relay server.

The fileTimestamp field was added to the FileContentCache class.

In RelayEndpoint, the fileTimestamp is now considered when checking the cache. A cached response is only used when its file timestamp exactly matches the current timestamp of the file on the filesystem.