The previous use of Java's default serialization dumped
all implementation-detail class names and fields into the serialization,
making it basically impossible to improve the implementation.
Two strategies here:
- prohibit serialization of unresolved configs, which are
the location of a lot of implementation detail
- delegate all serialization to an Externalizable
SerializedConfigValue class, which serializes
using fields that have lengths. Unknown fields
can thus be skipped and we can write code to
support obsolete fields, and so on.
As a side effect, this makes the serialization far more compact
because we don't need the Java per-class header noise, and we
jump through some hoops to avoid writing out duplicate ConfigOrigin
information. It still isn't super-compact compared to something
like protobuf but it's a lot less insane.
