Add missing lock around dubb buffer Write
Add missing lock around dubb buffer Write

Fixes a bug that would cause data coming out of dagr to occasionally be
all stompy. This is fine at lower volumes because you don't notice it,
but it can quickly result in very wrong messages being sent to
InfluxDB.

Change-Id: Ia0f94cb361e2f9256a111bf1f0592ea8c6a2c685

nilium/dagr