Deprecate carbonsearch support?

[Civil]

For the content: carbonsearch was the booking.com's way to implement tags without implementing tags on go-carbon side: https://github.com/kanatohodets/carbonsearch

It only stored current repesentation of tags and used a special query syntax on top of normal graphite query syntax.

I'm not sure if carbonsearch ever had any adoption outside of Booking.com environment and as they've switched to their own fork of carbonapi, I'm no longer sure it's worth to maintain support for that in carbonapi.

So my current plan is to add deprecation note (likely message with severity Error during startup, but not fatal one) and ask for input on if it's used. And if there will be no response until let's say November 2020, I'm going to remove support for carbonsearch from the codebase.

Main reason is that currently I'm not testing it at all and I'm not even sure if it still in working state.