crash if expires_ms is negative

[danieleds]

Here:

arq/arq/connections.py

Lines 162 to 174 in 9d7944b

	enqueue_time_ms = timestamp_ms()
	if _defer_until is not None:
	score = to_unix_ms(_defer_until)
	elif defer_by_ms:
	score = enqueue_time_ms + defer_by_ms
	else:
	score = enqueue_time_ms
	expires_ms = expires_ms or score - enqueue_time_ms + self.expires_extra_ms
	job = serialize_job(function, args, kwargs, _job_try, enqueue_time_ms, serializer=self.job_serializer)
	pipe.multi()
	pipe.psetex(job_key, expires_ms, job)

If expires_ms is negative (score - enqueue_time_ms + self.expires_extra_ms < 0) then we get a crash:

redis.exceptions.ResponseError: Command # 1 (PSETEX arq:job:677f11b1fbaf40c48b656cf35054851b -2505600000 ��g}�(�t�N�f��XXXX��a��)XXXXXXX����k�}��et���>��u.) of pipeline caused error: ("invalid expire time in 'psetex' command",)

This can happen when setting a negative _defer_by, as suggested here to give the job higher priority.

If the user specified a negative _defer_by, the way the score is computed should change so that the expires_ms is never negative or zero