DAOS-333 include: Export raft_become_follower()

Move the declaration of raft_become_follower() from raft_private.h to
raft.h, so that users may voluntarily give up leadership.

Signed-off-by: Li Wei <wei.g.li@intel.com>

Author: liw

include/raft.h

@@ -691,6 +691,10 @@ int raft_apply_all(raft_server_t* me_);
  * losing it's consensus guarantees. */
 void raft_become_leader(raft_server_t* me);
 
+/** Become follower. This may be used to give up leadership. It does not change
+ * currentTerm. */
+void raft_become_follower(raft_server_t* me);
+
 /** Determine if entry is voting configuration change.
  * @param[in] ety The entry to query.
  * @return 1 if this is a voting configuration change. */

include/raft_private.h

@@ -75,8 +75,6 @@ int raft_election_start(raft_server_t* me);
 
 int raft_become_candidate(raft_server_t* me);
 
-void raft_become_follower(raft_server_t* me);
-
 void raft_randomize_election_timeout(raft_server_t* me_);
 
 /**