**Description**
Fix two issues:
1. the self.session_started[1] is set as true too early for exceptional case.
2. the session is added too early[2] before session is actually started.

Enhanced the magic kernel by adding one internal magic to allow retry session
creation. Currently, the session creation error is treated as fatal error which
make it impossible[3] to retry after fixing the endpoint info.

[1] https://github.com/jupyter-incubator/sparkmagic/blob/bfabbb39a0249197c2c05c8efe681710fff9151b/sparkmagic/sparkmagic/kernels/kernelmagics.py#L357
[2] https://github.com/jupyter-incubator/sparkmagic/blob/bfabbb39a0249197c2c05c8efe681710fff9151b/sparkmagic/sparkmagic/livyclientlib/sparkcontroller.py#L90
[3] https://github.com/jupyter-incubator/sparkmagic/blob/bfabbb39a0249197c2c05c8efe681710fff9151b/sparkmagic/sparkmagic/kernels/kernelmagics.py#L350

Please see the movitivation for our use cases.
**Motivation**
To enable user to choose an Spark cluster and connect to it at runtime(means the
sparkmagic configuration is not generated in advance), we need to configure the endpoint
after kernel starting and enable retry session creation after fixing the incorrect endpoint.

We discussed this with Alejandro(aggFTW@) and he suggested to leverage the following _do_not_call_*
methods to configure the endpoint dynamically and create session after specifying configuration.
1.    _do_not_call_delete_session
2.    _do_not_call_change_endpoint
3.    _do_not_call_start_session

To conditionally allow the session creation retry, the changes introduced a flag
to allow retry on previous fatal of session creation. By default, the behavior
is kept unchanged and do not allow retry on fatal.

**Testing Done**
1. Added unit test.
2. Also ran the code in a notebook to verify.

**Backwards Compatibility Criteria (if any)**
The enhancement is activated conditionally and no compatibility issue is expected.

**Description**
When session start() throws exception, session delete() is invoked when
session status is not in NOT_STARTED_SESSION_STATUS to clean up resource.
This makes it safe to add the session to session manager only when session
is started properly.

This change is to address the edge case which could cause leaking session.
From client side we could see timeout issue but actually the session is
already created on Livy side.

**Testing Done**
Added unit test cases.
Also tested in a notebook.