GH-163: Stage only the difference from a Record's last synchronized state on save

[jtnelson]

Summary

A save previously wrote a Record's entire remembered state, so an instance with a stale view erased changes that other writers committed after the instance was loaded (the seat and vault data loss in cinchapi/cinchapi-server#151 and cinchapi/cinchapi-server#145). A save now writes only the difference between the record's current state and the state it last loaded or saved.

Fixes #163.

Behavior

• A collection field and realm membership stage per-element adds and removes. Elements and realms that other writers added or removed concurrently survive the save, including when the instance empties the collection.
• A scalar field is written only when its value changed, and cleared only when a previously present value was removed. A field the instance never touched stages nothing, so a concurrent change to it survives. When two writers change the same scalar field, the later save still wins for that field.
• hasUnsavedChanges() reports whether a save would write anything. A mutation with no serialized effect (for example, reordering a List, since the database stores an unordered set of values) is not an unsaved change.
• A save is a statement of the changes the instance made, not of complete record state: re-saving a record does not repair stored data that drifted through writes made outside of Runway, and assigning a collection wholesale does not guarantee that storage exactly matches the assigned collection when other writers changed the stored set concurrently.
• Saver gained add and no longer has reconcile.
• Failed-save restore, beforeSave gating, save and delete listeners, preventStaleWrites, and the single-key atomic operations keep their existing contracts.

Scope

• Regression coverage is in GH163; its reproduction tests fail against the prior save semantics.
• The changelog entry sits under Version 3.0.0 (unreleased). The .version bump is deferred to release.