removed building knownAtoms table multiple times

Summary:
This improved diviner performance.

Before:

➜  phabricator $ time ../diviner/bin/diviner .
../diviner/bin/diviner .  35.14s user 21.88s system 99% cpu 57.326 total

After:

➜  phabricator $ time ../diviner/bin/diviner .
../diviner/bin/diviner .  14.77s user 6.60s system 98% cpu 21.692 total

Test Plan: run diviner and look on the doc.

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3527