First prototype of a timeline profiler

[szegedi]

A prototype implementation of a new profiler type ("timeline") that can be operated simultaneously with "wall" profiler to gather information suitable for presentation as a timeline from async_hooks. This one takes the coarser-grained approach and essentially summarizes durations of various async events in 10ms (actually, 1/99s for Datadog reasons) quanta. Another approach with full-fidelity activation information would be possible, but we'd need to start emitting the Go execution trace format for that.