Site: parallelise meter reading

[andig]

/cc @Herbert23 please give it a try, remember to test with the -race build to catch any concurrency problems.

Note this PR might swallow a secondary problem in the the errgroup if more than one error occurs and will only error on required measurements, i.e. power.

[Herbert23]

@andig build with race and tested with 6 TQ-EM420, one SMA Inverter and 4 Bender.
In ~10 min testing there was one i/o timeout an no other errors.

As far as i understand this PR "only" parallelises the call to the seperate usages of EMs, but every EM within one usage is still updated synchronously one after each other, isn´t it?

[andig]

That's true. Idea was that for the typical modbus meter the measurements would all be collected sequentially as long as you don't need to fallback on backoff. Next step would be parallelising the individual meters per category. For loadpoints that was already done.