Fix deadlock on shutdown by releasing the GIL during del

On delete rocksdb waits for the background thread to finish.
However the background threads needs the GIL to execute python-code
(for example comparator)
=>
* main thread has GIL
* main thread waits for background thread
* background thread tries to get GIL
which means deadlock

rocksdb/_rocksdb.pyx

@@ -1095,7 +1095,9 @@ cdef class DB(object):
 
     def __dealloc__(self):
         if not self.db == NULL:
+            with nogil:
                 del self.db
+
         if self.opts is not None:
             self.opts.in_use = False