Fixes RedisLabs#49

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 <[email protected]>

Fix travis old C build

Refactor

Remove malloc from entry appending

Added documentation to mention that user is responsible for managing
memory.

Added HOWTO guide

Header file cleanup