Fix cache issue in stream_rename

We saw a bug where renamed streams had missing data at the end.  I
think what happened is:

- Write data to /old/path
- Rename to /new/path
- Write data to /new/path
- Cache entry for /old/path gets evicted, file gets truncated

Instead, make sure we evict /old/path right away when renaming.

nilmdb/server/bulkdata.py

@@ -194,6 +194,9 @@ class BulkData(object):
         if oldospath == newospath:
             raise ValueError("old and new paths are the same")
 
+        # Remove Table object at old path from cache
+        self.getnode.cache_remove(self, oldunicodepath)
+
         # Move the table to a temporary location
         tmpdir = tempfile.mkdtemp(prefix = "rename-", dir = self.root)
         tmppath = os.path.join(tmpdir, "table")