SPDY: Add Slot field for SYN_STREAM (and also unused field...)

Ping-Bug: 10566
Change-Id: I4d3d991eb8b5ede349a361d1f6b605b4a67a98b6
Reviewed-on: https://code.wireshark.org/review/4645
Reviewed-by: Michael Mann <mmann78@netscape.net>