Shrink camtypes.BlobMeta a bit.  Saves 4 bytes of memory per blob.

For me,

Before:
Corpus stats: 148.859 MiB mem: 799465 blobs (89.487 GiB) (282303 schema (6478 permanode, 150962 file (112224 image), ...)

After:
Corpus stats: 146.559 MiB mem: 799465 blobs (89.487 GiB) (282303 schema (6478 permanode, 150962 file (112224 image), ...)

Change-Id: Id421dfc9d72c2a24c228418c01f9a3f05900c941