Added verification of finished requests

[Ev3nt]

This code is more secure than the stub in https://github.com/FirebirdSQL/firebird/blob/master/src/dsql/DsqlRequests.cpp#L383

// Fetch from already finished request should not produce error
if (!(request->req_flags & req_active))
{
	if (req_timer)
		req_timer->stop();

	trace.fetch(true, ITracePlugin::RESULT_SUCCESS);
	return false;
}

It can be used as a second level of cursor status checking.

[dyemanov]

A few notes:

• The original goal is to avoid fetching beyond EOF in cursors, which is currently still possible in the remote protocol prefetch logic. This causes isc_req_sync errors being reported in the trace. It can be additionally fixed inside the protocol code, but this patch is more robust from the possible regressions POV.
• While this protection it's not strictly required in master (due to the aforementioned check added by Dimitry Sibiryakov recently), it's IMHO better understandable.
• It unifies EOF handling between uni-directional and scrollable cursors.
• It can be easily cherry-picked into the prior versions, if desired.