Make copydb work with SASL (SCRAM)

copydb with sasl begins with an admin command, "copydbsaslstart", followed by a call to "copydb" and a second call to "copydb". All commands are done on the target server's "admin" database.

CLIENT TO SERVER: {

    copydbsaslstart: 1,

    fromhost: "host",

    fromdb: "db",

    mechanism: "SCRAM-SHA-1",

    payload: BinData

}

The payload is the same as for "saslStart" with SCRAM-SHA-1. The server replies:

SERVER TO CLIENT: { conversationId: 1, done: false, payload: BinData}

There is no copydbsaslcontinue. Instead, copydb gets alternate parameters:

CLIENT TO SERVER: {

    copydb: 1,

    fromhost: "host",

    fromdb: "db",

    todb: "db2",

    payload: BinData,

    conversationId: N

} 

The payload is the same as for saslContinue with SCRAM-SHA-1. The conversation continues like:

SERVER TO CLIENT: { conversationId: 1, done: false, payload: BinData, ok: 1.0 }

CLIENT TO SERVER: { copydb: 1, fromhost: "host", fromdb: "db", todb: "db2", payload: BinData, conversationId: 1 }

SERVER TO CLIENT: { conversationId: 1, done: true, payload: BinData, ok: 1.0 }

The database has now been copied.

The shell helper copyDatabase has a new "mechanism" parameter that can be "MONGODB-CR" or "SCRAM-SHA-1". If no mechanism is provided, the shell helper uses SCRAM-SHA-1 if the target server has maxWireVersion >= 3 and MONGODB-CR otherwise.