[ntcore] NT4 stabilization and upgrades (tracking issue)

[PeterJohnson]

• Connection timeouts (use WS ping/pong?) ([ntcore] Require periodic ping/pong to detect disconnect more quickly #5223)
• Only sync timestamps once (at initial connection) so they aren't affected by traffic ([ntcore] Only measure network latency/exchange timestamps on initial connection #5224)
• Rate limit initial connection data interchange (Glass cyclically disconnects in NT4 mode #5262)
• Clients subscribe to meta-data so publishing operations are more subscriber-driven ([ntcore] Use subscriber metadata when publishing client values #4694)
• Add option for not retaining last value ([ntcore] Add property for server saving last value #5220)
• Implementation: undo pimpl'ing, break into multiple files as appropriate ([ntcore] Remove pimpl from implementation internals #5221)
• Value storage optimization ([ntcore] Optimize Value storage for string/raw/arrays #5219)
• Unit tests
• Make local storage smart about network subscribers to only enqueue data to network when needed
• Make server call back to local storage for current values on new connection? (interrelated with rate limiting initial connection data interchange)
• Avoid artificial subscription to all on server local storage; this adds an extra copy on every set for no reason. Instead build this into the server.
• Reduce memory usage (in particular some buffer pools like WebSocketConnection's don't have overall size constraints, so hold onto peak usage) ([ntcore] Limit buffer pool size to 64KB per connection #5485)
• Add interface for measuring network traffic
• Process updates more fairly from multiple clients (don't batch process)
• Change networking implementation away from libuv? Likely needed to improve backpressure characteristics
• Type mismatch: don't update / return lastValue (so GetValue() doesn't return garbage)
• Type mismatch: show via metatopics that a subscriber or publisher has a mismatch and is thus inactive? Include the type string?
• Type mismatch: provide API access to determine that a mismatch occurred (and thus the publisher/subscriber is inactive)

The first one (and maybe the second one as well) requires a protocol version bump (to avoid breaking older clients that don't follow the requirements).

[PeterJohnson]

Closing in favor of managing this via a project. https://github.com/orgs/wpilibsuite/projects/13