Add shutdown method to Flare Client API

[YuanTingHsieh]

Issue

• Flare API does not close the Flare agent and CellPipe resources
• LauncherExecutor does not shutdown its threadpoolexecutor
• SubprocessLauncher have a racing issue when multiple threads try to stop/start the process and monitor the process

Description

• Adds the shutdown functionality to the Flare API.
• In addition, introduces the context concept so prepare for future if multiple connections need to be made.
• Existing API usage is still compatible.
• Makes corresponding changes to tracking and lightning API
• Adds shutdown of threadpoolexecutor in LauncherExecutor
• Adds a lock in SubprocessLauncher

New API Usage:

import nvflare.client as flare
from nvflare.client import FLModel

with flare.init() as ctx:
    input_model = flare.receive(ctx=ctx)
    # do some training
    flare.send(FLModel(xxx), ctx=ctx)

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[yanchengnv]

See my comments and questions.

[yanchengnv]

We need to be able to shutdown the API.

[yanchengnv]

See my comments for corrections and improvement.

[YuanTingHsieh]

/build

[yanchengnv]

LGTM