mongodbreader may should not always use `MULTIPLE` mode

when mongodbreader connect to db server it use :
`public MongoClient(List seeds, List credentialsList)`
construction method to instance MongoClient.
this will cause client use MULTIPLE mode to make up cluster settings.
`Cluster created with settings {hosts=[10.7.x.x:30016], mode=MULTIPLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}`

In my scenario，mongodb was run on k8s with REPLICA_SET mode, this will cause client keep trying to touch backend service:
```
Server 10.7.x.x:30016 is no longer a member of the replica set. Removing from client view of cluster.
Canonical address mongodb-1.mongodb-headless.bangwo8-db-prod.svc.cluster.local:27017 does not match server address. Removing 10.7.x.x:30016 from client view of cluster
Updating cluster description to {type=REPLICA_SET, servers=[{address=mongodb-0.mongodb-headless.bangwo8-db-prod.svc.cluster.local:27017, type=UNKNOWN, state=CONNECTING}, {address=mongodb-arbiter-0.mongodb-arbiter-headless.bangwo8-db-prod.svc.cluster.local:27017, type=UNKNOWN, state=CONNECTING}, {address=mongodb-1.mongodb-headless.bangwo8-db-prod.svc.cluster.local:27017, type=UNKNOWN, state=CONNECTING}]
```
finally datax job will faild with java.net.UnknownHostException.

but if I change to use construction method :
`public MongoClient(ServerAddress addr, List credentialsList)`
the MongoClient will use SINGLE to make up cluster settings. And then I can search data normally.

Start at the mongodbreader code that constructs MongoClient and compare the two constructors shown in the issue, then reproduce the connection against a replica set using the reported server configuration. Done means the reader connects without repeatedly removing replica-set members or failing with UnknownHostException.

Written by the indexing model from the issue text.