Support unlimited pending checkpoints

Prevents the spinning that used to happen if RunCheckpoint was called
with 3 pending checkpoints. This spinning was done when holding
thread_list_lock_ and thread_suspend_count_lock_ and could deadlock
if any of the pending checkpoints required any of these locks.

The fix is to use an overflow list instead of having a fixed limit of
3.

Changed suspend stress test to have more threads and only compare last
line since there may be libbacktrace spam like:
"+E/libbacktrace(69891): void SignalHandler(int, siginfo_t *, void *):
Timed out waiting for unwind thread to indicate it completed."

Bug: 28988206

Change-Id: I2ae611506147d5199d59a08eee0395f7fa35d448