nginx-tfs new features and bug-fixs.

1. fix nginx do not close connection bug
2. use readv2 if read from start
3. write failed now retry ns instead of ds
4. fix do not remove dirty block cache when stat large file bug
5. fix pread custom file bug when read offset is larger than 2 times of segment's start offset
6. fix rc timer bug
7. fix write failed bug when data to write is in memory
8. fix select cluster bug
9. add Content-type response header
Merge pull request #329 from zhcn381/upstream2

nginx-tfs new features and bug-fixs.