Current bucket_metainfos table schema was far from optimal. All
operations are using project_id and name to find bucket but primary key
was on id field which is completely not used. Turns out that secondary
index was not as fast as it could be because each select was also joining
primary index which was slowing down whole query.
Primary key is changed to project_id/name. Other indexes are dropped
and id column is now nullable which gives us space for dropping it
completely from schema and code later.
Change-Id: I477056a4243d39e39489f1473de01ded1ed24bbb
