ability to drop a measurement

__Use case:__
At this point if a measurement contains a reasonable amount of measurements, there seems no way to drop the measurement.
I'm currently experiencing this on a measurement I briefly created to build a field<->series relationship.
This measurement can no long be deleted (and I guess deleting the measurement that was subject to this investigation can never be dropped as it collects 350 million values a day).

__Proposal:__
Let drop measurement run in background such that it is not subject to query timeouts.

__Current behaviour:__
drop measurement will timeout

__Desired behaviour:__
drop measurement will be executed in background and the result is reported in a metrics that can be acquired via http://.../debug/vars

__Alternatives considered:__
are there any?

Start by tracing the measurement-drop operation and reviewing the HTTP /debug/vars entry point mentioned in the proposal. Done means drop measurement can run in the background without query timeouts and reports its result through metrics available via /debug/vars.

Written by the indexing model from the issue text.