Infra: fix AOSP->GitHub mirror bot

1. Introduce a per-CL size limit that avoids pushing CLs that are
   too big and would be refused by github (e.g. aosp/945483)
2. Avoid pushing CLs that are too old. Hard code a min CL number
   and only push CLs greater than that.
3. Make most git invocations concurrent. It hides fork()+exec()
   and I/O latency.

The union of all the above makes the sync cycle last 10-30 seconds
instead of 5-10 minutes.

Test: manual
Change-Id: I15a1e361b5327c48df3d4b2ddc0b2e1571782c76