Fix memgraph crashing on exit

Summary:
Snapshot scheduler object was released from unique ptr and not actually freed, which
caused the snapshooter to access the tx_engine after it was destructed.

Reviewers: mferencevic

Reviewed By: mferencevic

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1325

src/database/graph_db.cpp

@@ -271,7 +271,7 @@ PublicBase::~PublicBase() {
   tx_engine().LocalForEachActiveTransaction(
       [](auto &t) { t.set_should_abort(); });
 
-  snapshot_creator_.release();
+  snapshot_creator_ = nullptr;
   if (impl_->config_.snapshot_on_exit) MakeSnapshot();
 }