[bug] drop/delete terribly slow

### Bug report

Influx version: **v1.5.1**
OS version: **Ubuntu Linux 17.04**

__Steps to reproduce:__
1. Have a big database with > 20000 series
2. Try to execute `drop series from /{{regexp_pattern}}/`.
3. or try to execute `drop measurement /{{regexp_pattern}}/`.
4. or try to execute `delete from /{{regexp_pattern}}/`.

It will take > 30 minutes.

__Expected behavior:__
1. Well deleting data should take miliseconds?
2. Or there should be some special command modifier that will do it fast (I think this is becaus esome influx DB cleaning stuff). And there should be another comment to execute this "long" part manually, so for example I can delete ton of series and run cleanup process manually (to reclaim space) from a cron job or something like this.