Batch commits

[CyrusVorwald]

session.commit() should be called as bulk operations outside of loops, such as in balanced_backend/cron/pool_stats.py

[robcxyz]

Most of these commits that are in line with the session.add/merge were from debugging / have little performance penalty in general.

RE pool_stats / commit outside loop - these loops generally are way too big to commit that much data outside of it. Would have memory issues.