Summary:
* On DB reopen, iterate all SST file's properties and sum up live data size of all blob files to use as GC stats.
* Remove previous logic to set GC mark on blob files on reopen.
* Refactor `OnFlushCompleted` and `OnCompactionCompleted`. Check file state is `kPendingLSM` before transit file state. 
* Refactor `BlobFileMeta`.

Test Plan:
Updated unit tests
Signed-off-by: Yi Wu <yiwu@pingcap.com>