Merge trunk

* Small fixes around provider loading added: tests/mock_engine.h
* Use a fusion engine provider to generate higher quality position
  updates added:
  include/location_service/com/ubuntu/location/fusion_provider.h
  include/location_service/com/ubuntu/location/fusion_provider_selecti
  on_policy.h
  include/location_service/com/ubuntu/location/newer_or_more_accurate_
  update_selector.h
  include/location_service/com/ubuntu/location/newer_update_selector.h
  include/location_service/com/ubuntu/location/update_selector.h
  src/location_service/com/ubuntu/location/fusion_provider.cpp
  src/location_service/com/ubuntu/location/fusion_provider_selection_p
  olicy.cpp
* Enable dual landings according to https://wiki.ubuntu.com/citrain/LandingProcess#Dual-landing_for_stable_and_devel:
  * Both vivid+overlay and xenial packages are built from the same
    source.
  * To account for differences in major revisions (necessary to handle
    the g++ 5 ABI break), we generate install files on the fly.
* Lower number of concurrent clients to stabilize acceptance test on
  loaded builders.
* No change rebuild.
* No change rebuild.
* libubuntu-location-service3: Replaces libubuntu-location-service2,
  config file /etc/gps.conf in both packages.
* Bump major revision and fix LP:#1478750 to account for toolchain update.
* Log to stderr by default, relying on upstart to rotate logs
  appropriately. (LP: #1447110)
* Add SntpClient for querying reference time information.