allow graceful shutdown in asyncengine

[Ivansete-status]

Description

• This is meant to avoid leaking file descriptors when using Nim libraries.
• Currently, nim-chronos global dispatcher delegates the de-allocation of certain resources to the OS, when an app ends.
• Nim libraries leverage nim-chronos threads, i.e., a Nim library component starts a nim-chronos thread to operate with.
• Certain use cases, e.g. tests, require start/stop Nim library component, which implies start/stop nim-chronos threads.
• Then, certain file descriptors are leaked upon consecutive Nim-library-component creation/destruction (nim-chronos thread start/stop.) For example, on unix-like systems, fds handled by selector are leaked.

Changes

• Attend and complete in-flight work requests to the chronos thread.
• nim-chronos thread blocks further tasks when shutdown is in progress.
• Free all resources, on each platform, when all tasks are completed.

Context

The issue has been detected while testing nim-sds in status-go

This status-go PR contains better issue detail.
This temporary workaround was implemented.