DROP MEASUREMENT "xyz" not working

### Bug report

Cannot delete measurments.
I accidentally created a bunch of MS SQL server measurements into my Telegraf db, and want to remove them. When i try to remove them with DROP MEASUREMENT "" it doesnt actually seem to drop it...

My mistake seems to have created over 1000 measurements, so i suspect the bug/issue is due to the number of measurements.
If i create a new test database i can drop measurements as expected.

__System info:__
Influx 1.5.1 running on Red Hat 4.8.5-11

__Steps to reproduce:__

Create a lot of measurements - i did this by trying to use the MS SQL Server telegraf input.

1. USE telegraf
2. DROP MEASUREMENT "xa7_sessions"
It doesnt matter what measurement i specify, i cannot remove it. There is no error
3. SHOW MEASUREMENTS
Its still there!

__Expected behavior:__
Id expect the measurement to no longer be returned after running the DROP MEASUREMENT command.

__Actual behavior:__
SHOW MEASUREMENT continues to return the measurement..and will not die ;-(

__Additional info:__ [Include gist of relevant config, logs, etc.]