Update client IceProtocolConnection to schedule first heartbeat only …

[bernardnormier]

…once connected

This is a small fix to the "send heartbeat" logic in IceProtocolConnection.

Prior to this PR, an Ice client connection could send heartbeats before receiving the initial ValidateConnection frame from the server. That's a minor violation of the ice protocol that neither Ice nor IceRPC would detect. It's also very unlikely. Nevertheless, a small bug.

I also added a test to verify that an ice client connection that doesn't write anything is not aborted by the server connection idle monitor.

[bernardnormier]

On macos CI, the new test failed with:

  Failed IceRpc:Tests:IceProtocolConnectionTests:Server_idle_monitor_does_not_abort_connection_when_client_does_not_write_anything [530 ms]
  Error Message:
     Assert.That(serverShutdownRequested.IsCompleted, Is.False)
  Expected: False
  But was:  True

  Stack Trace:
     at IceRpc.Tests.IceProtocolConnectionTests.Server_idle_monitor_does_not_abort_connection_when_client_does_not_write_anything() in /Users/runner/work/icerpc-csharp/icerpc-csharp/tests/IceRpc.Tests/IceProtocolConnectionTests.cs:line 580
   at IceRpc.Tests.IceProtocolConnectionTests.Server_idle_monitor_does_not_abort_connection_when_client_does_not_write_anything() in /Users/runner/work/icerpc-csharp/icerpc-csharp/tests/IceRpc.Tests/IceProtocolConnectionTests.cs:line 586
   at NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.BlockUntilCompleted()
   at NUnit.Framework.Internal.MessagePumpStrategy.NoMessagePumpStrategy.WaitForCompletion(AwaitAdapter awaiter)
   at NUnit.Framework.Internal.AsyncToSyncAdapter.Await[TResult](Func`1 invoke)
   at NUnit.Framework.Internal.AsyncToSyncAdapter.Await(Func`1 invoke)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.RunTestMethod(TestExecutionContext context)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.Execute(TestExecutionContext context)
   at NUnit.Framework.Internal.Execution.SimpleWorkItem.<>c__DisplayClass3_0.<PerformWork>b__0()
   at NUnit.Framework.Internal.ContextUtils.<>c__DisplayClass1_0`1.<DoIsolated>b__0(Object _)

1)    at IceRpc.Tests.IceProtocolConnectionTests.Server_idle_monitor_does_not_abort_connection_when_client_does_not_write_anything() in /Users/runner/work/icerpc-csharp/icerpc-csharp/tests/IceRpc.Tests/IceProtocolConnectionTests.cs:line 580

530 ms is quite long. Maybe an 100 ms idle timeout is too short, especially since the fixture runs the tests in parallel.