Make DirectMonotonicReader.Meta more compact

[original-brownbear]

Even with the all-zero bit-length optimization recently introduced, these Meta instances tend to consume a non-trivial amount of heap still. We can do away with almost 40% of their size by removing the offsets array that can be calculated on the fly when creating the readers which translates into tens of MB of heap or more for some ES use cases with high segment + field count. Also added a singleton DirectMonotonicReader instance for the common all-zero-values that we have a Meta singleton for already.