refactor: Get rid of more redundant chain methods

This just drops three interfaces::Chain methods replacing them with other calls.

Motivation for removing these chain methods:

- Need to get rid of findFirstBlockWithTimeAndHeight for #10102, which doesn't
  support overloaded methods
- Followup from
  https://github.com/bitcoin/bitcoin/pull/16426#discussion_r412487403
- phantomcircuit comments about findNextBlock test
  http://www.erisian.com.au/bitcoin-core-dev/log-2020-06-06.html#l-214

Behavior is not changing in any way here. A TODO comment in
ScanForWalletTransactions was removed, but just because it was invalid (see
https://github.com/bitcoin/bitcoin/pull/19195#discussion_r448020762), not
because it was implemented.

src/interfaces/chain.h

@@ -44,6 +44,10 @@ public:
     FoundBlock& time(int64_t& time) { m_time = &time; return *this; }
     FoundBlock& maxTime(int64_t& max_time) { m_max_time = &max_time; return *this; }
     FoundBlock& mtpTime(int64_t& mtp_time) { m_mtp_time = &mtp_time; return *this; }
+    //! Return whether block is in the active (most-work) chain.
+    FoundBlock& inActiveChain(bool& in_active_chain) { m_in_active_chain = &in_active_chain; return *this; }
+    //! Return next block in the active chain if current block is in the active chain.
+    FoundBlock& nextBlock(const FoundBlock& next_block) { m_next_block = &next_block; return *this; }
     //! Read block data from disk. If the block exists but doesn't have data
     //! (for example due to pruning), the CBlock variable will be set to null.
     FoundBlock& data(CBlock& data) { m_data = &data; return *this; }
@@ -53,6 +57,8 @@ public:
     int64_t* m_time = nullptr;
     int64_t* m_max_time = nullptr;
     int64_t* m_mtp_time = nullptr;
+    bool* m_in_active_chain = nullptr;
+    const FoundBlock* m_next_block = nullptr;
     CBlock* m_data = nullptr;
 };
 
@@ -77,9 +83,9 @@ public:
 //!   wallet cache it, fee estimation being driven by node mempool, wallet
 //!   should be the consumer.
 //!
-//! * The `guessVerificationProgress`, `getBlockHeight`, `getBlockHash`, etc
-//!   methods can go away if rescan logic is moved on the node side, and wallet
-//!   only register rescan request.
+//! * `guessVerificationProgress` and similar methods can go away if rescan
+//!   logic moves out of the wallet, and the wallet just requests scans from the
+//!   node (https://github.com/bitcoin/bitcoin/issues/11756)
 class Chain
 {
 public:
@@ -90,11 +96,6 @@ public:
     //! any blocks)
     virtual Optional<int> getHeight() = 0;
 
-    //! Get block height above genesis block. Returns 0 for genesis block,
-    //! 1 for following block, and so on. Returns nullopt for a block not
-    //! included in the current chain.
-    virtual Optional<int> getBlockHeight(const uint256& hash) = 0;
-
     //! Get block hash. Height must be valid or this function will abort.
     virtual uint256 getBlockHash(int height) = 0;
 
@@ -102,13 +103,6 @@ public:
     //! pruned), and contains transactions.
     virtual bool haveBlockOnDisk(int height) = 0;
 
-    //! Return height of the first block in the chain with timestamp equal
-    //! or greater than the given time and height equal or greater than the
-    //! given height, or nullopt if there is no block with a high enough
-    //! timestamp and height. Also return the block hash as an optional output parameter
-    //! (to avoid the cost of a second lookup in case this information is needed.)
-    virtual Optional<int> findFirstBlockWithTimeAndHeight(int64_t time, int height, uint256* hash) = 0;
-
     //! Get locator for the current chain tip.
     virtual CBlockLocator getTipLocator() = 0;
 
@@ -130,11 +124,6 @@ public:
     //! information.
     virtual bool findFirstBlockWithTimeAndHeight(int64_t min_time, int min_height, const FoundBlock& block={}) = 0;
 
-    //! Find next block if block is part of current chain. Also flag if
-    //! there was a reorg and the specified block hash is no longer in the
-    //! current chain, and optionally return block information.
-    virtual bool findNextBlock(const uint256& block_hash, int block_height, const FoundBlock& next={}, bool* reorg=nullptr) = 0;
-
     //! Find ancestor of block at specified height and optionally return
     //! ancestor information.
     virtual bool findAncestorByHeight(const uint256& block_hash, int ancestor_height, const FoundBlock& ancestor_out={}) = 0;