Add possibility for GroupValueRead at startup

[nomisim]

This adds a go routing that performs the startup read operation. It is inspired by the poller go routine.

The startup read routine sends GroupValueRead telegrams to all group addresses configured with ReadStartup: true (the default is false). By default the GroupValueRead telegrams are sent in a 200ms interval, in order to not everwhelm the bus. The interval can be changed by adopting the ReadStartupInterval: 200ms configuration parameter in the global section of the config file.

If this is acceptable I can also update README.md to have it documented - just let me know.

Resolves #54

[chr-fritz]

First of all, a big thank you for the PR. Apart from the three minor change requests, I have two major ones before I merge the PR:

First, please build suitable tests for the most important parts, Run() and getMetricsToRead(config *Config) GroupAddressConfigSet, from the startupReader and second, add new configuration options to README.md.

[nomisim]

This took a while now, as I was on vacation in between.

However, I managed to update the code today:

• integrated the fixes you proposed
• updated the README.md to include new config options
• addedd tests for Run() and getMetricsToRead(config *Config) GroupAddressConfigSet from the startupReader
• merged with upstream

Hope this goes into the right direction - let me know what you think.

[chr-fritz]

No problem, I hope the holiday was relaxing.

Thanks for the feature, I'll merge itl.

However, I won't build a new release just yet, because I still want to implement the new connection options myself (especially TCP tunnels).

[nomisim]

Thanks, sure no problem. I am looking forward to the TCP feature as well.