ffldb: close block files

On methods readBlock and readBlockRegion, the opened files were never
closed which resulted in errors when attempting to delete these files on
pruning on windows.  Properly closing the files avoids this error.

database/ffldb/blockio.go

@@ -524,6 +524,7 @@ func (s *blockStore) readBlock(hash *chainhash.Hash, loc blockLocation) ([]byte,
 	serializedData := make([]byte, loc.blockLen)
 	n, err := blockFile.file.ReadAt(serializedData, int64(loc.fileOffset))
 	blockFile.RUnlock()
+	blockFile.file.Close()
 	if err != nil {
 		str := fmt.Sprintf("failed to read block %s from file %d, "+
 			"offset %d: %v", hash, loc.blockFileNum, loc.fileOffset,
@@ -584,6 +585,7 @@ func (s *blockStore) readBlockRegion(loc blockLocation, offset, numBytes uint32)
 	serializedData := make([]byte, numBytes)
 	_, err = blockFile.file.ReadAt(serializedData, int64(readOffset))
 	blockFile.RUnlock()
+	blockFile.file.Close()
 	if err != nil {
 		str := fmt.Sprintf("failed to read region from block file %d, "+
 			"offset %d, len %d: %v", loc.blockFileNum, readOffset,