upnp: refactor to improve handling of port mappings renewal

The two main changes are:

1) Renewal requests are now sent for both UPnP and NAT-PMP mappings, not
   just NAT-PMP. The old code asked for an infinite lifetime when
   creating UPnP mappings and assumed that it got it, but this is not a
   safe assumption. (See GitLab issue below for more information.)

2) The updateMappingList function was removed. This function used to be
   called every 30 seconds to handle a bunch of unrelated tasks (one of
   which was renewing port mappings) and generated mostly unnecessary
   network traffic every time when using UPnP (because of the call to
   pruneMappingList). These tasks are now performed separately instead
   of being bundled together, and only when needed (either based on a
   timer or on certain events occuring, depending on the task).

GitLab: #31
Change-Id: Id0f60ddb76fb8eb4517eadbb971892d125cebfc7