If a reconnect is triggered on a built-up connection the current
espconn instance is replaced without waiting for the invocation of
its disconnection callback.
As a result the disconnection callback gets invoked when the new
espconn instance has already been set, falsely terminating the newly
created connection.
Fix this by disconnecting the current connection cleanly before trying
to reestablish the connection. This change also prevents reconnections
via MQTT_Connect. Instead users should disconnect the current
connection via MQTT_Disconnect before invoking MQTT_Connect for an
existing client.
Tuan PM