New, and simplified, (PersistitStore)SchemaManager metadata storage

Remove use of GrowableByteBuffer in ProtobufReader and Writer and generally clean up that flow.

Remove SerializationType from the SchemaManager flow. Lots of cruft for something that hasn't been needed in over a year and no plans to use.

New metadata layout for PSSM. The current is a bit creaky and while it could support the new needs (e.g. "byProgress" range), we're making other incompatible changes so we might as well clean it up. Note that this rejects old volumes explicitly.

The new metadata on FDB is essentially identical, substituting manual prefixes for directories.

Changes are in support of online DDL (touched on in the new Javadoc) and, if desired, this can wait for subsequent branches.