Fix freeflow couplingmanager issue

The singleton approach (static thread_local) fails as soon as one has two instances of a coupling manager in the same program. The occurring error is very hard to debug. Since the singleton failed to make the coupling manager thread safe, we go back to mutable caches.

(Note that theoretically we could work with locks to make it thread-safe but this has been tested to introduce large overhead. Global caches will need to be eliminated eventually. In the meantime, we at least make it work for multiple couplingmanager instances.)

Fixes #1419 (closed)