Previously RPC server shutdown procedure listened to the execution
channel and stopped at the first element that arrived in the queue. This
could lead to the following problems:
* stopper could steal the execution result from subscriber
* stopper didn't wait for other subscription actions to complete
Add dedicated channel to `Server` for subscription routine. Close the
channel on `handleSubEvents` return and wait for signal in `Shutdown`.
Signed-off-by: Leonard Lyubich <leonard@morphbits.io>
465d3f43d2