Enable resume of partially downloaded utxo-sets

closes #718

contrib/FastSync/load-utxo-set.sh

@@ -66,7 +66,7 @@ cd "$TAR_DIR"
 IS_DOWNLOADED=false
 if [ ! -f "$TAR_FILE" ]; then
   echo "Downloading $UTXO_DOWNLOAD_LINK to $TAR_FILE"
-  wget "$UTXO_DOWNLOAD_LINK" -q --show-progress
+  wget "$UTXO_DOWNLOAD_LINK" -c -O $TAR_FILE.partial -q --show-progress && mv $TAR_FILE.partial $TAR_FILE
   IS_DOWNLOADED=true
 else
   echo "$TAR_FILE already exists"
@@ -117,4 +117,4 @@ if docker volume inspect generated_bitcoin_wallet_datadir &>/dev/null; then
   echo -e '\033[33mHow to proceed: If you agree to delete your Bitcoin Core wallet, run "docker volume rm generated_bitcoin_wallet_datadir"\033[0m'
 else
   echo "You can now run btcpay again (btcpay-up.sh)"
-fi
+fi