Allow rocksdb.DB instances to be manually closed

While `delete rocks_ptr` is a deterministic operation in C++, the Python analogue `del rocks_handle` is not – disposal and finalization of the Rocks database are entirely dependent on the Python garbage collector (q.v. the `python-rocksdb` tests themselves, which call `gc.collect()` after `del rocks_handle` to attempt to force the destructor to run). This change exposes a method to trigger the destruction of the underlying Rocks database pointer (deterministic!) through the Python Rocks handle; existing code will not need to be changed, as the Python object destructor (non-deterministic!) will now call this method.

This is the second revision of this PR – it resolves the first revision, stephan-hof#39.

rocksdb/_rocksdb.pyx

@@ -1671,6 +1671,9 @@ cdef class DB(object):
         self.opts.in_use = True
 
     def __dealloc__(self):
+        self.close()
+
+    def close(self):
         cdef ColumnFamilyOptions copts
         if not self.db == NULL:
             # We have to make sure we delete the handles so rocksdb doesn't