[hail] Fix performance problems in aggregators by binding arguments

[tpoterba]

oops

[tpoterba]

on a not-particularly-adversarial example:
Master:

2019-08-29 17:59:57,197: INFO: [1/1] Running table_aggregate_int_stats...
2019-08-29 18:00:09,871: INFO:     burn in: 12.67s
2019-08-29 18:00:21,455: INFO:     run 1: 11.58s
2019-08-29 18:00:32,744: INFO:     run 2: 11.29s
2019-08-29 18:00:45,349: INFO:     run 3: 12.60s
2019-08-29 18:00:57,343: INFO:     run 4: 11.99s

PR:

2019-08-29 18:01:48,767: INFO: [1/1] Running table_aggregate_int_stats...
2019-08-29 18:01:58,227: INFO:     burn in: 9.46s
2019-08-29 18:02:05,939: INFO:     run 1: 7.71s
2019-08-29 18:02:13,240: INFO:     run 2: 7.30s
2019-08-29 18:02:21,290: INFO:     run 3: 8.05s
2019-08-29 18:02:29,386: INFO:     run 4: 8.09s
2019-08-29 18:02:37,365: INFO:     run 5: 7.98s