BUG-5626: Move leaderId/leaderPayloadVersion fields

Handling of getLeaderId() depends on behavior:
- for (Isolated)Leader, it is always its local ID
- for Candidate it is always null
- for Follower it can vary
The same is true of leader payload version.

This patch moves these two fields to Follower, makes Candidate return constants and
makes Leader return the values stored in the local context.

Change-Id: I0fca58ab985f14460eef9647a89ede02242e6e7c
Signed-off-by: Robert Varga <rovarga@cisco.com>