switch_id: consistently use SDN_Types.switchId

Various other equivalent types were being used in its place, including
VInt.t, SDN_Types.fieldVal, and SDN_Types.header_val.
switch_id: use int64 for for switchId

Switch ids are 64 bits across versions, so just use that.
Merge pull request #243 from frenetic-lang/switch_id

 Use int64 for switchId type