Make clusto_query faster. Changes:

 - Rewrites how initial pools are filled to only query a subpool once. This
   is didn't used to matter, but now we have way more pools than servers.
 - Default to not merging container attributes
 - Adds a basic optimizer to execute anything that isn't an Attribute query
   first
remove dead code
Make clusto_query faster. Changes:

Summary:
 - Rewrites how initial pools are filled to only query a subpool once. This
   is didn't used to matter, but now we have way more pools than servers.
 - Default to not merging container attributes
 - Adds a basic optimizer to execute anything that isn't an Attribute query
   first

Time to query `pool = api and attr system.cpucount = 24`:
 - In old clusto-query: 2 minutes 47 seconds
 - In new clusto-query with -m flag passed: 24 seconds
 - In new clusto-query without -m flag passed: 7 seconds

Reviewers: dchen, xiaojian, evan

Reviewed By: evan

Differential Revision: https://code.uberinternal.com/D41634