This allows starting and stopping separately each protocol server
without restarting ClickHouse.
This also allows adding or removing `listen_host` entries, which
start and stops servers for all enabled ports.
When stopping a server, the listening socket is immediately closed
(and available for another server).
Protocols with persistent connections try to wait for any currently
running query to finish before closing the connection, but idle
connection are closed quickly (depending on how often the protocol
is polled).
An extra ProfileEvent is added, `MainConfigLoads`, it is
incremented every time the configuration is reloaded. This helps
when trying to assess whether the new configuration was applied.
* Refactoring: part 1
* Refactoring: part 2
* Handle request using ReadBuffer interface
* Struggles with ReadBuffer's
* Fix URI parsing
* Implement parsing of multipart/form-data
* Check HTTP_LENGTH_REQUIRED before eof() or will hang
* Fix HTTPChunkedReadBuffer
* Fix build and style
* Fix test
* Resist double-eof
* Fix arcadian build
