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This plumbs through multi advertising capabilities for the
instance. Instances can register and unregister advertisements.

Now with virtual destructors to prevent memory leaks in the
StructTraits conversion.

BUG=637171,653338
TEST=use nrf connect on remote device and verify that packets
  registered from the instance are visible on the remote device,
  run tests with ASAN/LSAN turned on and verify there are no
  memory errors

Review-Url: https://codereview.chromium.org/2394683007
Cr-Commit-Position: refs/heads/master@{#423672}