Sync titan directory when adding blob files (#136)

Summary:
When adding new blob file, titan directory need to be fsync, otherwise files can be lost after power outage.

Test Plan:
run `strace -yfe fsync ./titan_titan_db_test --gtest_filter=*.Snapshot` and see titan directory being fsync-ed in the output.

Signed-off-by: Yi Wu <yiwu@pingcap.com>