Interface to an user event loop implementation for http\Client::configure()'s $use_eventloop option.
This interface was added in v2.6.0, resp. v3.1.0.
Initialize the event loop.
Run the loop as
long as it does not block.
Run the loop.
Register (or deregister) a socket watcher.
Register a timeout watcher.
Wait/poll/select (block the loop) until events fire.