Add streaming parser

[andig]

Fix #19

This PR adds a streaming reveicer and parser to handle RCT messages asynchronously. It has the following changes:

• connection continuously reads incoming data
• data is continuously being parsed by channel-based ParseAsync
• async parser adds support for long messages that didn't fit the buffer size before
• responses are cached for use with the synchronous Get api (to be removed?)
• the synchronous Query/Get api is complemented by channel-based Subscribe/Unsubscribe
• connection cache is removed- it's no longer needed
• legacy sychronous Parser is deprecated

TODO

• deduplicate legacy parser code
• decide async api removal (including the cache)
• make broker private
• make QueryXYZ connection methods global

Since this PR breaks the existing API, it should be behind a v2 version.

[andig]

@mlnoga wdyt regarding v2? If that's still an option for you we could finally move v2 to evcc-io and make the breaking change via new url.

[andig]

ping @mlnoga any thoughts?