CLOUDSTACK-4855: Throttle based on the # of outstanding requests to the directly managed HV host (direct agents)
Cloudstack sends requests to directly managed HV hosts (direct agents) using the direct agent thread pool. The size of the pool is determined by global config direct.agent.pool.size defaulted to 500.

Currently there is no restriction on the number of threads a direct agent can use from this shared thread pool to send requests to the host. This is fine as long as the host is responding to requests
in a reasonable amount of time. But if there is a considerable delay in getting response, the thread remain blocked for that much time. As more commands are send to the slow host threads keep getting
blocked. This can eventually lead to a situation where requests to healthy hosts cannot be processed as there are not enough free threads.

The problem being addressed here is to localize the impact of few bad hosts, so that entire management server is not affected.

One such way is to throttle based on the # of outstanding requests on per host basis. The outstanding requests to a host will be a % of direct agent pool size. This is configurable based on
direct.agent.thread.cap. The default value is 0.1 or 10%, a value of 1 would mean the old behavior where there is no upper cap. This will ensure that the impacted host will be bound by a upper cap on the number of threads it can use to process requests and not the entire pool.