[sgen] Staging of finalizer registrations.

[sgen] Staging of finalizer registrations.

Registering a finalizer required the GC lock to be taken.  To remove
this need we introduce an array where finalizer registrations are
staged in a lock-free way.  Only when the array (currently holding
1024 elements) is full is the GC lock taken and the finalizers are
inserted into the hash.  Of course this also needs to happen when a
collection starts and when a domain is unloaded.