reshard: small fix and cleanup

reshard: small fix and cleanup

At the end of each stage, finish() will be called once to guarantee
all entries can be flushed to dst shards. This may costs long time
if the counts of dst shards is vast, especially in second stage,
so renew reshard_lock is needed.

Signed-off-by: Mingyuan Liang <liangmingyuan@baidu.com>