Previously scanMultipartDir() returns object info channel and timer
channel where timer channel is used to check whether object info
channel is alive or not. This causes a race condition that timeout
may occur while object info channel in use.
This patch fixes the issue by removing timer channel and uses object
info channel directly where each object info has End bool field
indicates whether received object info is end or not.
- Rename dir.go as 'fs-multipart-dir.go'
- Move the push/pop to fs-multipart.go and rename them as save/lookup.
- Rename objectInfo instances in fs-multipart as multipartObjInfo.
