Low-memory mode disabled

Previous binary release tarballs (from v0.9.3 through v0.9.10) were built with DVID_LOW_MEMORY=1, which forced badger databases to operate in a special low-memory mode. This is not desirable for production uses. Release builds published here on our github releases page had sub-optimal performance. Our build procedure has changed to avoid using DVID_LOW_MEMORY in release builds.

Other changes

• add update of max label on label index ingestion; fixes #352
• return 404 instead of 204 for non-existant key in keyvalue instance; fixes #353
• allow changing branch names as part of flatten-metadata rpc command
• fix possible race condition on labelsz test, which might lead to test fails on resource-limited build machines