sheep: fix a build error

Reported-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
optimize epoch_log structure to reduce network and memory

Current epoch_log contains a long nodes array to sync nodes and
epoch in the cluster. It is simple, but there is a potential
performance issue because each epoch log occupies nearly 500
KBytes. If the cluster members change frequently, epoch is lifted
frequently, more and more unused data is transfered between
client and server. If we don't find a way, the performance will
go from bad to worse.

Although the max node number is 6144, we only use a few of them.
Therefore, the first solution is using a zero-length array,
client (dog) and server (sheep) will negotiate an appropriate
supported node number. This way will spend much less memory and
will run much faster than before.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>