Drop redundant CpuBoundWork usage in JsonRpcConnection::Disconnect()

[yhabteab]

Although there is locking involved here, it shoudln't take too long for the thread to actually acquire it, since there aren't that many threads dealing with endpoint clients concurrently. It's just wasting pointless time trying to obtain a CPU slot.

refs #9988

[Al2Klimov]

It's not as trivial as you might think. Endpoint::RemoveClient calls Endpoint::OnDisconnected which calls EndpointDbObject::UpdateConnectedStatus which calls DbObject::OnQuery. But this just enqueues stuff – should be fine.

[julianbrost]

Endpoint::RemoveClient calls Endpoint::OnDisconnected which calls EndpointDbObject::UpdateConnectedStatus which calls DbObject::OnQuery. But this just enqueues stuff – should be fine.

Also, the corresponding Endpoint::AddClient() does pretty much the same and is doing it without CpuBoundWork already.