fix(http): time out idle connections at the connection limit

[SeriousCoding789]

What does this PR do?

• Time out idle HTTP connections once node.maxHttpConnectNumber (default 50) is reached: HttpService sets ConnectionLimit.setIdleTimeout(10_000).
• Add ConnectionLimitTest. The idle timeout can be set through a @VisibleForTesting setter, so the test runs with 1 second instead of 10.

Why are these changes required?

At the connection limit Jetty stops accepting, and a connection that never sends a request holds its slot until the connector's 30-second idle timeout has fired twice (Jetty half-closes an idle connection before closing it), about 60 seconds. Meanwhile other HTTP / JSON-RPC clients are locked out. The new timeout applies only while the limit is reached and frees such a slot in about 20 seconds.

This PR has been tested by:

• Unit Tests: ConnectionLimitTest passes in about 2 seconds and fails with a read timeout when the setIdleTimeout call is removed; SizeLimitHandlerTest, HttpApiAccessFilterTest, HttpInterceptorMetricsTest and checkstyle pass.

Follow up

None.

Extra details

• No config key or default changes; HTTP behavior below the connection limit is unchanged.
• While the limit is reached, Jetty applies the timeout to every open connection, not only to connections that have sent nothing: a keep-alive client that stays idle for more than 10 seconds between two requests on one connection is disconnected and has to reconnect. Time spent processing a request does not count as idle, so requests in progress are not affected.
• Does not cover clients that trickle bytes to keep a connection active.
• Split out of fix: harden start.sh #7003.