Cancel http read only queries if client socket goes away

[nvartolomei]

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

Category (leave one):

• Improvement

Short description (up to few sentences):

Cancel http read only queries if client socket goes away

Detailed description (optional):

To check the socket status, try to read one byte from socket in a
non-blocking way:
0 - client closed the connection
>= 1 - client send more data, we are ignoring this case for now
timeout - normal case, client is waiting for response
... - socket broken?

Dirty, but should do the job. Limiting to readonly queries as I don't
want to mess with alter queries / insert select and others.

Closes #1403

[nvartolomei]

Hopefully poco doesn’t support pipelining, this may break things *.

[nvartolomei]

Can Socket#poll be used for this without abusing read and making too many assumptions about how Poco handles sockets?

[alexey-milovidov]

Let's write a test with cancellation of infinite query like SELECT count() FROM system.numbers.

[nvartolomei]

@alexey-milovidov the test is in, let me know if it works.

re readonly guard, I didn't want to alter the behaviour for pending inserts, alters or any other similar queries. Although this better be handled in other places (it might be actually), I wanted to be extra cautious. (also, initially it was implemented without _PEEK flag).

re poll, I tried if (socket->poll(Poco::Timespan(), SelectMode::ERROR)) cancelQuery() but it does not pass the test on macOS, haven't checked other OS. Maybe this is not the correct way of doing it, I'm not very familiar with these things.