Implement eloop for Fuchsia.

Use the mtl message loop to implement hostap's eloop interface. This
implementation only includes registering and cancelling timeouts. The
other features are not necessary for wpa_supplicant.

Accidentally submitted and reverted.
Previous-Change-Id: I7260f15547d28133d33aaa9467b0eb6e920ee79d

Change-Id: I34f01e70da8e0fc184b84b2dccff367b88c15ae2