Netty servers: handle requestTimeout properly, add idleTimeout

[kciesielski]

Fixes #3795

This PR replaces usage of ReadTimeoutHandler with IdleStateHandler.
The ReadStateHandler was configured using NettyConfig.requestTimeout value, but it wasn't serving the purpose of failing when there is no response within given timeout. This handler throws an exception when there is no data read , which means no request from the client.

We will be using IdleStateHandler to deal with timeouts correctly:

1. One such handler is created for each request, with timeout set to nettyConfig.requestTimeout. If it expires, the channel is closed, a HTTP 503 response is returned, and in-flight requests in the channel are canceled. An error is logged.
2. if backend manages to produce a response, the handler is removed.
3. Thus, in case of pieplining, there will be multiple IdleStateHandler instances running, one for each running request processing.
4. Additionally, we introduce NettyConfig.idleTimeout. Similarly to idle-timeout in akka-http, it defines a time of no reads nor writes, after which the channel will be closed silently (only with a debug log). This is a "normal situation" that may happen after a long-running keep-alive connection expires (typically from a web browser), like in io.netty.handler.timeout.ReadTimeoutException logged for a simple endpoint #3795, which initiated this work.
5. The value of idleTimeout is set by default to 60s, while requestTimeout is 20s.