Fix CONNECT for large MQTT payloads

[brentru]

The remaining length used by connect is incompatible with large payloads (>127 bytes). An instance of this is when a long (> 300 byte) password, such as a JSON Web Token, or a long client identifier is provided.

• The remaining length calculation used by this library has been removed in favor of using the algorithm described by the MQTT V3.1.1 specification (https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718030).
• Default keepalive value sent by connect() increased to 60 seconds to ensure the MQTT broker rejects incorrect connections, instead of hanging _wait_for_msg().
• Fixed header bytearray removed, switching to a variable bytearray.
• Variables within connect renamed to correct names per the MQTT spec.

Tested with a PyPortal running CircuitPython Latest, two tests were run to verify compatibility with existing (small) payloads and large connect payloads...

MiniMQTT Example - Adafruit IO WiFi - testing small (<127 byte) connect payload.

Connecting to Adafruit IO...
25346.9: DEBUG - Creating new socket
25347.0: DEBUG - Attempting to establish secure MQTT connection...
appemnding...
25349.8: DEBUG - Sending CONNECT to broker
25349.9: DEBUG - Fixed Header: bytearray(b'\x10>\x00')
Variable Header: bytearray(b'\x04MQTT\x04\xc2\x00<')
25350.2: DEBUG - Receiving CONNACK packet from broker
Connected to Adafruit IO! Listening for topic changes on brubell/feeds/onoff

Connecting to Google's Cloud IoT MQTT Server - testing large (512B+) connect payload.

Attempting to connect to mqtt.googleapis.com
25400.9: DEBUG - Creating new socket
25400.9: DEBUG - Attempting to establish secure MQTT connection...
25403.5: DEBUG - Sending CONNECT to broker
25403.5: DEBUG - Fixed Header: bytearray(b'\x10\xb6\x04\x00')
Variable Header: bytearray(b'\x04MQTT\x04\xc2\x00<')
25403.8: DEBUG - Receiving CONNACK packet from broker
Connected to MQTT Broker!
Flags: 0
 RC: 0