Provide better implementation for FileStream::id()

[mikee47]

This PR provides a better ID string for file streams.

How ETAGs are generated is not specified. Previous implementation was like this:

m_snprintf(buf, ETAG_SIZE, _F("00f-%x-%x0-%x"), stat.id, stat.size, stat.name.length);

Issues are:

• on some architectures compiler warns about differing types %x vs arguments
• name can change without length being affected; if name changes then file lookup would fail anyway
• modification timestamp is a good metric, should be incorporated