Pass the expected remaining time to tasks

[Kobzol]

For some tasks it might be useful to know what is the expected time for which they might run (for example so that they can shorten their run or do more checkpointing). We could pass an environment variable (e.g. HQ_REMAINING_TIME) with the expected remaining time of the worker in seconds (if it is known).

[spirali]

Just noting obvious: it should be technically min(remaining_worker_time, task_time_limit).

[spirali]

Btw: Maybe we can named it "HQ_REMAINING_SECONDS" to make it clear what are the units.