Move update_checkpoints() to a later stage

update_checkpoints() makes a few DNS requests and can take up to 20-30 seconds to complete (3-6 seconds on average). It is currently called from core::handle_incoming_block() which holds m_incoming_tx_lock, so it blocks all incoming transactions and blocks processing while update_checkpoints() is running. This PR moves it to until after a new block has been processed and relayed, to avoid full monerod locking.

tests/unit_tests/node_server.cpp

@@ -72,6 +72,7 @@ public:
   bool get_test_drop_download_height() const {return true;}
   bool prepare_handle_incoming_blocks(const std::vector<cryptonote::block_complete_entry>  &blocks_entry, std::vector<cryptonote::block> &blocks) { return true; }
   bool cleanup_handle_incoming_blocks(bool force_sync = false) { return true; }
+  bool update_checkpoints(const bool skip_dns = false) { return true; }
   uint64_t get_target_blockchain_height() const { return 1; }
   size_t get_block_sync_size(uint64_t height) const { return BLOCKS_SYNCHRONIZING_DEFAULT_COUNT; }
   virtual void on_transactions_relayed(epee::span<const cryptonote::blobdata> tx_blobs, cryptonote::relay_method tx_relay) {}