Add bounds checking to Entry::current_running_duration

[stuhood]

Problem

As shown in #6640, once we moved Entry locks outside of the Graph lock in #6095, it became possible for heavy_hitters to race against Nodes that are starting outside of the Graph lock.

Solution

Add a bounds check for Instant::duration_since in current_running_duration, to handle the case where a Node is started after we begin computing heavy_hitters. While switching to Instant::elapsed would "mostly" avoid this case, it would incur additional syscalls, without fully avoiding panics.

Result

Fixes #6640.