curl: Remove unnecessary explicit calls to internal event handler

Remove calls to curl_multi_do where the relevant handles are already
registered to the event loop.

Ensure that we kick off socket handling with CURL_SOCKET_TIMEOUT after
adding a new handle.

Signed-off-by: Matthew Booth <mbooth@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>