Feature: don't calculate asic_job_frequency but measure it.

[mutatrum]

Taken from Slack:

As a test, I extended the job_frequency to 5000ms, which is too long, and I see this:

I (18251) ASIC_task: Send next job: 20000000 Nonce 00000000 0000000000000000 diff 10000
I (18241) stratum_task: setup message accepted
I (19341) bm1368Module: Job ID: 18, Core: 45/1, Ver: 12AE2000
I (19341) asic_result: Ver: 32AE2000 Nonce DE4C035A diff 417.7 of 10000.
I (19651) bm1368Module: Job ID: 18, Core: 52/13, Ver: 1807A000
I (19651) asic_result: Ver: 3807A000 Nonce BC080368 diff 288.5 of 10000.
I (21201) bm1368Module: Job ID: 18, Core: 45/1, Ver: 12AE2000
I (21201) asic_result: Ver: 32AE2000 Nonce DE4C035A diff 417.7 of 10000.
I (21511) bm1368Module: Job ID: 18, Core: 52/13, Ver: 1807A000
I (21511) asic_result: Ver: 3807A000 Nonce BC080368 diff 288.5 of 10000.
I (23061) bm1368Module: Job ID: 18, Core: 45/1, Ver: 12AE2000
I (23061) asic_result: Ver: 32AE2000 Nonce DE4C035A diff 417.7 of 10000.

It starts looping, it returns the same 2 nonces of that range, every 1860ms.

This gives me an idea, instead of trying to calculate exactly what this period needs to be, we can just feed it a job for a few seconds, and time how long it took the chip to loop around. No need for difficult calculations, works on all chips and all nonce range masks and such.

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The 1860ms is awfully close to the 1812ms from #420 Is the measurement wrong, or is there some time measurement overhead? It seems - in this small sample - to be consistent.