[Deliverable] Allow graceful shutdown on nim-chronos

[chair28980]

Summary

The current nim-chronos design assumes some resources, such as selector file descriptors in unix-like systems, are freed only when the process ends. This is not the case for nim-ffi where any library may eventually need to start/stop several instances of the library within the whole process lifetime.

The issue was detected while testing nim-sds within status-go tests. See the sub-issue.

[igor-sirotin]

logos-messaging/nim-ffi#4 doesn't need a separate dedicated Deliverable and Milestone, moved it under #194.