Bug 827157 - Robust and performance sensitive recording of sessions; r=rnewman

We have introduced a new background service that captures session state
in preferences. Firefox Health Report now moves entries from preferences
to its database at payload generation time.

We've also introduced a few random APIs, such as enqueueTransaction()
and the ability for providers to have access to their own pref branch.
Bug 827157 - Part 2: report current session information. r=rnewman

The previous patch regressed this because HealthReporter was talking
directly to storage instead of going through the measurement instance.
Merge m-c to s-c.
Merge m-c to inbound.
Bug 823872 - Add test for OS.Constants.{libc, Win}. r=Yoric