Idea: Use a manager thread to support full C++ multi-threading

[eschnett]

I am calling Julia from C++ in kotekan, independent of jluna. (I was not aware of jluna when I implemented this.) kotekan is multi-threaded.

To support multi-threading, kotekan is using the following approach:

• To initialize Julia, a new C++ thread ("Julia manager thread") is created
• There is a global variable containing a task queue
• The Julia manager thread waits for tasks in the queue and executes them
• When a thread wants to call Julia, this call is placed onto the queue

A "task" is a std::function<void(void)> wrapping a closure. A std::future is used to return the value from Julia to the calling thread.

See https://github.com/kotekan/kotekan/blob/1b930517ab9fbc71cbc02a787af197a316627a09/lib/core/juliaManager.hpp and https://github.com/kotekan/kotekan/blob/1b930517ab9fbc71cbc02a787af197a316627a09/lib/core/juliaManager.cpp for the implementation in kotekan. A similar implementation would be possible for jluna.