Batch the accumulations to only transmit every so often, so we don't incur
too much in the way of IPC overhead penalties.
What this doesn't do:
* remove or restructure child telemetry code to adapt to the new way
* send the telemetry anywhere
* allow for the child process to clear child histograms
* support anything but histograms (but this is expected and okay)
MozReview-Commit-ID: JnUkcmN3Ya7
