GCS PC-remerge code can't handle remerge of multiple PCs about galera

[dirtysalt]

here we use prim_seqno as component id. Usually we could tell which component is the last primary component by comparing last_seqno(received) field, however this field could be zero if no action is received and applied, and there might be situation that different components with same uuid:seqno but different prim_seqno.

However when we remerge, we have to make sure that there is no other component with higher prim_seqno but same uuid:seqno. So we have to make sure when remerge, the representative candidate(rc), found == prim_joined. And here is a unit test show current implementation might be faulty. de8eb93#diff-76c47081af4860b648dac639f8985611R485