Make Zebra's tip estimate match zcashd exactly

[teor2345]

Motivation

Zebra's chain tip estimate is slightly different to zcashd's.

We don't think this will cause any issues, but if it does, they will come up in tests like:

• Add lightwalletd full sync integration test #3511
• ci: run a full lightwalletd sync from a close-to-tip zebrad #3926

Specifications

Here is what the zcashd code does:

1. If zcashd has stopped syncing, return the current height: https://github.com/zcash/zcash/blob/6cd5b8792bbe0a3cc1064e1884914415f0c6e7d7/src/rpc/blockchain.cpp#L1068-L1071
2. If the block time is ahead of the current time, return the current height: https://github.com/zcash/zcash/blob/f7006e9a33ca37ae88520e434ee5c2b2498c1cc6/src/metrics.cpp#L141-L143
3. Otherwise, estimate the tip height: https://github.com/zcash/zcash/blob/f7006e9a33ca37ae88520e434ee5c2b2498c1cc6/src/metrics.cpp#L145-L160

Zebra APIs

Here's the SyncStatus method that returns if we're close to the tip:

zebra/zebrad/src/components/sync/status.rs

Lines 46 to 47 in 5d7f986

	/// Check if the synchronization is likely close to the chain tip.
	pub fn is_close_to_tip(&self) -> bool {

Here's the buggy NetworkChainTipHeightEstimator code. Removing negative times should simplify it a lot.
(The bug fix will probably make some tests fail, Janito can help fix them.)
https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/chain_tip/network_chain_tip_height_estimator.rs#L111-L120

Tasks

Here's one way to do this fix:

1. Move SyncStatus to zebra_node_services
2. Add SyncStatus to RpcImpl, and use it to check if we're close to the tip
3. Fix NetworkChainTipHeightEstimator to return the current height if the block time is ahead of the current time

Related Work

This ticket was split out of:

• feat(rpc): Implement getblockchaininfo RPC method #3891 (comment)

[teor2345]

I've been running tests with lightwalletd, and it keeps on running without this fix.

We can re-open this ticket if we discover that it is actually needed during testing.