This code has caused a long history of problems. This change rewrites it
completely with something that is, hopefully, much simplier and robust
and adds more testing.

Change-Id: Ibeef51f9386afd95d5b73316e451eb3a2d7ec4e0
Reviewed-on: https://boringssl-review.googlesource.com/8033
Reviewed-by: Adam Langley <agl@google.com>

Change-Id: Ia7f4f4f6d063d882cf3d3ac0f5f33ad8d8cd9875
Reviewed-on: https://boringssl-review.googlesource.com/5151
Reviewed-by: Adam Langley <agl@google.com>