storage: commit from time to time when truncating

This is all the more important for RocksDB that it wants to keep all
transaction work in RAM.

Once we had to truncate 40% of a 1TB MyRocks DB with 24 partitions,
4 being processed in parallel. Even when committing between partitions,
MariaDB used up to 200 GB. Without the commit, 1TB RAM would not have
been enough.

neo/storage/database/manager.py

@@ -14,7 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import os, errno, socket, struct, sys, threading
+import os, errno, socket, struct, sys, threading, time
 from collections import defaultdict
 from contextlib import contextmanager
 from copy import copy
@@ -853,7 +853,12 @@ class DatabaseManager(object):
             assert tid, tid
             cell_list = []
             my_nid = self.getUUID()
+            commit = 0
             for partition, state in self.iterAssignedCells():
+                if commit < time.time():
+                    if commit:
+                        self.commit()
+                    commit = time.time() + 10
                 if state > tid:
                     cell_list.append((partition, my_nid, tid))
                 self._deleteRange(partition, tid)