We need to access the data in stat to get the process start time, so we
can calculate the time from process start until application.init for the
frameworkStart probe.
This replaces the StartupTaskManager we had with a more general class.
New implementation is a thread-safe "gated task executor", which either
runs the task right away if it's marked as 'ready', or queries it to be
executed later on.
This ability to either execute or queue a task will be useful later on in the
commit series.
This monitor for hot start was intended to be used by FNPRMS to measure
hot start. However, hot start was deprioritized so it's now essentially
unused.
As mentioned in the class comment, we could use an alternative
implementation to measure perf from outside the application however
it's more complicated to set up and less consistent with our cold
startup methodology so we'll start with this.
Michael Comella