Need a command-line flag to allow changing HBase scanning behavior

[tsuna]

Reference discussion: Understanding query performance on the mailing list.

We need a command-line knob to tweak the number of rows HBase scanners fetch at once. The default value is a good estimate but can lead to sub-optimal performance on very sparse tables (lots of very small rows).

[anomalizer]

I am living with this less than ideal patch for now
InMobi/opentsdb@c01cf6b30b7cbedb1490

There seems to be no way to configure Scanner properties given a HbbaseClient. We would either have to update asynchbase project to support this or add a helper function to tsdb that returns a pre-configured scanner and use it everywhere. Both seemed like a lot of code, so I a relying on environment variables and just updated the query code

[anomalizer]

Changed implementation to work off command line arg instead of environmental variable
InMobi/opentsdb@f4f4af5