BuildMutable generates a catalog.MutableDescriptor from scratch based on: - the original descriptor (optional), - the mutated descriptor, - an MVCC timestamp (optional) to set the mutated descriptor's ModificationTime with.

FromBytesAndMVCCTimestamp is very similar to FromSerializedValue but does not feature a tag in the bytes slice.

FromSerializedValue deserializes a descriptor from a *roachpb.Value and properly sets its timestamps based on the MVCC timestamp before returning it as a builder if it's not empty.