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Change the start_link API for emqx_client

Prior to this change, emqx_client:start_link does 2 works in one call:
- init an erlang process for emqx_client
- send MQTT CONNECT to remote broker

But this solution have some drawbacks:

- the return value of `start_link` compiles the return values of the 2
 works: {ok, Pid, MqttResult}. It is inconsistent with the return value
 of `gen_statem:start_link`, may causes confusions.

- the return mode of the 2 works are different:
  `start_link` should always return {ok, Pid} or {error, Reason}, but
 connecting to mqtt may throw out exceptions as it handles the
 socket. But the caller couldn't have thought of the exception, he would
 pattern match on the result of `emqx_client:start_link`, but it crashed!

- If the init work succeed but the connection failed, the caller couldn't
get a Pid from the return value, but indeed it was created inside the
emqx_client. This hides the fact that the Pid was created, and when the
Pid dies, the caller would receive an message from a Pid it doesn' know about.

This change divived these 2 work into 2 APIs:
- `start_link/1` is to build and verify the options, and returns {ok,Pid}
 (on success) or {error, Reason} (on failure).
- `connect/1` is to send MQTT CONNECT, and returns {ok, MQTTResult::properties()} or
 {error, MQTTReason}. MQTT reason codes will contains in the `MQTTReason`.

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