give a chance for the last obj to retry

[JacksonYao287]

when install snapshot happens , if something off (for example, data crc mismatch)happens to the last obj, we have no chance to ask leader to resend the last obj, since

NuRaft/src/handle_snapshot_sync.cxx

Line 520 in 99eeef3

if (is_last_obj) {

will definitely apply_snapshot when is_last_obj is true.

this change give a chance for follower to ask leader to resend the last obj. for example , if follower find the data crc mismatch, it can set is_last_obj to false , which means this is not the last obj the follower wants. meanwhile , follower can set obj_id to the last_obj_id, so that leader will resend the last obj to follower