I left current master running in production for an hour. When I returned, CPU utilization had jumped from 70% to 400%. perf top showed that the majority of time was being spent in Http2ConnectionState::restart_streams. Connected with debugger and two threads were spinning in that method. In each case this->stream_list.head->link.next == this->stream_list.head->link.prev and this->thread_list.head->link.next != NULL.
I assume that was are missing some locking and the stream_list is being manipulated by two threads in parallel.