plugins/resman: Fix memory leak, 2DA crash, move to 2.8 API.

* Convert deprecated hash_map to std::unordered_map, so
  small dependency on C++11 support is required.
* Move structures/api to 2.8 API.
* Add cache for NCS data files, which aren't normally released by
  NWN.  This cache is shared by all event consumers.
* CRes2DA requires it's m_bLoaded member to be set to 0 to properly
  initialize.
* Add additional members to ResManDemandResStruct to allow communicating
  resource type and modification time.  Modify members to avoid unsigned/
  signed warnings.
* Add ResourceExists event to query event consumers as to whether they
  contain the file question.