fil_space_t::acquire_low(): Introduce a parameter that specifies
which flags should be avoided. At all times, referenced() must not
be incremented if the STOPPING flag is set. When fil_system.mutex
is not being held by the current thread, the reference must not be
incremented if the CLOSING flag is set (unless NEEDS_FSYNC is set,
in fil_space_t::flush()).

fil_space_t::acquire(): Invoke acquire_low(STOPPING | CLOSING).
In this way, the reference count cannot be incremented after
fil_space_t::try_to_close() invoked fil_space_t::set_closing().

If the CLOSING flag was set, we must retry acquire_low() after
acquiring fil_system.mutex.

fil_space_t::prepare_acquired(): Replaces prepare(true).

fil_space_t::acquire_and_prepare(): Replaces prepare().
This basically retries fil_space_t::acquire() after
acquiring fil_system.mutex.