Use a single `NodeManager` per thread (#7204)

Use a single `NodeManager` per thread (#7204)

This changes cvc5 to use a single `NodeManager` per thread (using
`thread_local`). We have decided that this is more convenient because
nodes between solvers in the same thread could be exchanged and that
there isn't really an advantage of having multiple `NodeManager`s per
thread.

One wrinkle of this change is that `NodeManager::init()` must be called
explicitly before the `NodeManager` can be used. This code can currently
not be moved to the constructor of `NodeManager` because the code
indirectly calls `NodeManager::currentNM()`, which leads to a loop
because the `NodeManager` is created in `NodeManager::currentNM()`.
Further refactoring is required to get rid of this restriction.