Brotli support for HTTP interfaces

[fandyushin]

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For changelog. Remove if this is non-significant change.

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• New Feature

Short description (up to few sentences):
Brotli support for HTTP interfaces:
Content-Encoding: br

[alexey-milovidov]

Looks good.
Need to add a test.

[fandyushin]

its seems that I'm done

[alexey-milovidov]

Ok. Now it is implemented for data insertion.

How hard is to implement BrotliWriteBuffer and to use it when enable_http_compression is enabled and client said that it accepts br? And probably to provide http_brotli_compression_level setting.

Do you want to implement it in next pull request?

[fandyushin]

I've fixed .gitmodules. It changes too often :)
I think its not complicated to implement write buffer. I can do it.