Fix generateChronologicalKey() for 32-bit machines

Summary: This method relies on 64-bit math being available, which isn't a safe
assumption. Use the builtin bc functions instead for arbitrarily large integers.

Test Plan: @skrul, can you apply this locally and let me know if it works?

Reviewers: skrul, hunterbridges, jungejason, nh, tuomaspelkonen, aran

Reviewed By: skrul

CC: aran, skrul, epriestley

Differential Revision: 912