replication: get rid of replica->pause_on_connect flag

replicaset_connect() leaves appliers that failed to connect within the
specified time period running. To prevent them from entering 'subscribe'
stage prematurely (i.e. before replicaset_follow() is called), we set
replica->pause_on_connect flag, which will force them to freeze upon
successful connection. We clear this flag in replicaset_follow(). This
juggling with flags looks ugly. Instead, let's stop failed appliers in
replicaset_connect() and restart them in replicaset_follow().

Follow-up #2958