Refine Shutdown logic

[sergiupopescu199]

Description

Based on tokio's documentation about graceful shutdown explore the available solutions and propose a simple logic of handling graceful shutdown of the tasks the Indexer has:

• Indexer checkpoint sync
• RestApi

An interesting solution could be to explore the task_tracker

[kodemartin]

There are some functional requirements that pertain to the shutdown implementation:

1. The current checkpoint should be successfully processed. That is, once the shutdown signal is received all the data of interest that belong to the current checkpoint should be persisted in the db. Then the sync service should stop.
2. No other checkpoint than the current should be processed.

This brings up another question to explore: How does iota-data-ingestion-core handle the shutdown signal sent from the sync process?

[shufps]

related:
#33