Fix false corruption error when reading from v1 blob file (#221) (#222)

To support dictionary compression we added a v2 blob file header to store extra flags. However, the check in BlobFileReader , when reading a v1 blob file, falsely asserted the file header is shorter than expected:
https://github.com/tikv/titan/blob/ecb5cba016096309cbd9b566bdc4ce62307527a0/src/blob_file_reader.cc#L103
https://github.com/tikv/titan/blob/ecb5cba016096309cbd9b566bdc4ce62307527a0/src/blob_format.h#L394
Refactoring the code to bypass the check to fix the issue.

The issue is introduced in https://github.com/tikv/titan/pull/189

. TiKV is affected when Titan is enabled and upgrade from pre-5.0 versions to >=5.0.0 versions. It will make TiKV fall in crash loop.

Also adding a titan_blob_file_dump tool to dump blob file content.

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