Move {Identifiable,Persistent,}Payload

Move {Identifiable,Persistent,}Payload

Payloads are really part of raft message specification, not some vague
sal-clustering-commons detail. They have a strong relationship with
AppendEntries and therefore really need to be co-located.

As a first step in dealing with AppendEntries sizing, move Payloads so
we can properly evolve them.

JIRA: CONTROLLER-2037
Change-Id: I069f311b1c1b211b8429a721e6b239e5e08ed813
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 534bf6f83465cc8a575b097c1e28fbb1f34d110a)
(cherry picked from commit a36505d4c697db33941a5899172fee92256aa0b8)