Datasource processes exit with the mapget process

[l-laura]

When the mapget process is killed with the SIGKILL, the datasource processes spun up by it continue running.

There should be a POST /heartbeat request from mapget to datasources. Datasource subprocesses read the MAPGET_DATASOURCE_PROCESS_TIMEOUT environment variable, which indicates the timeout window in seconds. If this variable exists, the process shuts down when a heartbeat has not been received from mapget in the specified time window.

We should add a --datasource-executable-timeout command line argument to mapget.

The datasources should reply to the /heartbeat request with their unique datasource nodeID. In the future, mapget should use the keep-alive reply to:

1. Check if a datasource process is still alive. If not, mapget reports it and removes worker threads for the datasource.
2. Validate that the datasource at the given endpoint has the same ID as the originally registered datasource.