ffldb: close block files before deleting them

The block files may be open when deleteFile is called.  This resulted in
files not being deleted and erroring out on windows.  Properly closing
the files closing the files avoids this error.

database/ffldb/db.go

@@ -1630,6 +1630,19 @@ func (tx *transaction) writePendingAndCommit() error {
 	// We do this first before doing any of the writes as we can't undo
 	// deletions of files.
 	for _, fileNum := range tx.pendingDelFileNums {
+		blockFile := tx.db.store.openBlockFiles[fileNum]
+		if blockFile != nil {
+			// Close the old file under the write lock for the file in case
+			// any readers are currently reading from it so it's not closed
+			// out from under them.
+			blockFile.Lock()
+			_ = blockFile.file.Close()
+			blockFile.Unlock()
+
+			delete(tx.db.store.openBlockFiles, fileNum)
+			delete(tx.db.store.fileNumToLRUElem, fileNum)
+		}
+
 		err := tx.db.store.deleteFileFunc(fileNum)
 		if err != nil {
 			// Nothing we can do if we fail to delete blocks besides