core: Set tcp keep-alive for server-side pgwire connections.
core: Set tcp keep-alive for server-side pgwire connections.

Sets tcp keep-alive interval on pgwire connections if
COCKROACH_SQL_TCP_KEEP_ALIVE > 0.
That's all we get to set through Go. under linux, that impacts both
the time to first keep-alive and the repeat interval. We don't get to
set the count of keep-alive packets until the connection is considered
dead.

This would get around #13823, without requiring client library support
for keep alive.
We need to figure out if there are downsides to something like this
though.

Testing would kind of suck too. I don't think we can extract this kind
of information from a client connection in progress.

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