swim: make incarnation struct

Traditional SWIM describes member version as incarnation -
volatile monotonically growing number to refute false gossips.
But it is not enough in the real world because of necessity to
detect restarts and refute information from previous lifes of an
instance.

Incarnation is going to be a two-part value with persistent upper
part and volatile lower part. This patch does preparations making
incarnation struct instead of a number.

Volatile part is called 'version.

Part of #4280