raft: Fix next_index in install_snapshot reply handling.

raft: Fix next_index in install_snapshot reply handling.

When a leader handles install_snapshot reply, the next_index for
the follower should be log_start instead of log_end, because there
can be new entries added in leader's log after initiating the
install_snapshot procedure.  Also, it should send all the accumulated
entries to follower in the following append-request message, instead
of sending 0 entries, to speed up the converge.

Without this fix, there is no functional problem, but it takes
uncessary extra rounds of append-requests responsed with "inconsistency"
by follower, although finally will be converged.

Signed-off-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>