[RFC] performance improvements

[andig]

On volkszaehler (volkszaehler/vzlogger#272 (comment)) it was mentioned that reading registers one by one is much slower than reading blockwise.

This leads to the following ideas for improving readout performance or lowering load:

• group device registers into groups that can be read at once
• refactor the mqtt rate parameter (or add another one) to not limit pushing readings to mqtt but instead defer reading from device if rate is too high (NOTE: this would affect all consumers, not only mqtt)

[andig]

This is actually more complex than expected:

• meter reads are of variable size (i.e. ALE3 reads mostly but not only single registers)
• some meters only support reading up to a certain number of registers en bloc (at least ALE3)

[andig]

#83 provides a basis from implementing this. Closing here.