Adds synchronization primitives inside the initialization function to ensure that when the function returns, everything is guaranteed to be initialized.

Fixes #3120

Most of the diff is just indenting the code that was already there. If you prefer, I can make a second function instead to avoid changing so many lines.

I've also opted by this approach instead of just adding synchronized to the method declaration because that would force synchronization well after everything has been initialized, reducing parallelization (and therefore performance).

The new INITIALIZING variable is necessary because certain function calls inside initialize also call initialize themselves, so moving INITIALIZED = true to the end of the method would enter infinite recursion.