[feature request] Continuous executor

[mildred]

Is your feature request related to a problem? Please describe.
Continuous executor executes only a single process and does not consumes resources for updates. it just pushes an update. To monitor the system in high load situations, this is very useful.

Describe the solution you'd like
An executor that does not fork+exec every time it needs to update itself

Describe alternatives you've considered
none

Additional context
Implementation is easy and I can make a PR