blockchain_db: do not throw on expected partial results getting keys

When scanning for outputs used in a set of incoming blocks,
we expect that some of the inputs in their transactions will
not be found in the blockchain, as they could be in previous
blocks in that set. Those outputs will be scanned there at
a later point. In this case, we add a flag to control wehther
an output not being found is expected or not.

src/cryptonote_core/blockchain.cpp

@@ -3540,7 +3540,7 @@ void Blockchain::output_scan_worker(const uint64_t amount, const std::vector<uin
 {
   try
   {
-    m_db->get_output_key(amount, offsets, outputs);
+    m_db->get_output_key(amount, offsets, outputs, true);
   }
   catch (const std::exception& e)
   {