Tweak connection handling for better failure detection

[bekarau]

Tweak import lines.

Make connection more serial, giving small default timeout; allows for better failure checking. The way it currently stands, the connection is backgrounded and not made until the next action is done making the exception handler around mongo_connect redundant.

This patch forces a command to happen immediately to ensure that there is a connection by the time mongo_connect() returns.

The default timeouts were also large, gave the limited ability to override timeouts on the check command line.

[warrenpnz]

Nice work.
A couple of things to maybe look at, at line 291, the user/password auth might need a try/except to catch an incorrect password like:

        if user and passwd:
            db = con[authdb]
            try:
              #if not db.authenticate(user, password=passwd):
              db.authenticate(user, password=passwd)
            except PyMongoError:
                sys.exit("Username/Password incorrect")

Also the pymongo docs now recommend not doing background connect rather using "connect=false" then doing a low usage command to initiate the connection (https://api.mongodb.org/python/current/migrate-to-pymongo3.html#mongoclient-connects-asynchronously) e.g:

        if pymongo.version >= "2.3":
            if replica is None:
                con = pymongo.MongoClient(host, port, ssl=useSSL, connectTimeoutMS=5000, connect=False)
            else:
                con = pymongo.MongoClient(host, port, ssl=useSSL, connectTimeoutMS=5000, connect=False, read_preference=pymongo.ReadPreference.SECONDARY, replicaSet=replica )
        else:
            if replica is None:
                con = pymongo.Connection(host, port, slave_okay=True, network_timeout=10)
            else:
                con = pymongo.Connection(host, port, slave_okay=True, network_timeout=10)
                #con = pymongo.Connection(host, port, slave_okay=True, replicaSet=replica, network_timeout=10)

        try:
          result = con.admin.command("ismaster")
        except ConnectionFailure:
          print("Server not available")
          sys.exit(1)