add proper support for SIGSTOP and SIGCONT (currently, on replica set primary can cause data loss)

Untar and put all files under MONGDB. Then execute MONGODB/my.py can reproduce the bug. It triggers the following sequence of events. start a replica set of three replicas: S1, S2, S3. Say S1 is primary, S2/S3 are backups insert x:1 into S1 Suspend (i.e. send the mongod process a SIGSTOP signal) S1 wait for S2/S3 to elect a new leader, say S2 insert x:2 (sent to S2) with w=2 Resume (i.e. send the mongod process a SIGCONT signal) S1 S2 and S3 roll back x:2! But x:2 is supposed to be durable!