mining: ensure witness commitment check in submitBlock

When an IPC client requests a new block template via the Mining interface,
we hold on to its CBlock. That way when they call submitSolution() we can
modify it in place, rather than having to reconstruct the full block like
the submitblock RPC does.

Before this commit however we forgot to invalidate
m_checked_witness_commitment, which we should since the client brings a
new coinbase.

This would cause us to accept an invalid chaintip.

Fix this and add a test to confirm that we now reject such a block.
As a sanity check, we add a second node to the test and confirm that will
accept our mined block.

Note that the IPC code takes the coinbase as provided, unlike the
submitblock RPC which calls UpdateUncommittedBlockStructures() and adds
witness commitment to the coinbase if it was missing.

Although that could have been an alternative fix, we instead document that
IPC clients are expected to provide the full coinbase including witness
commitment.

src/interfaces/mining.h

@@ -54,9 +54,22 @@ public:
     virtual std::vector<uint256> getCoinbaseMerklePath() = 0;
 
     /**
-     * Construct and broadcast the block.
+     * Construct and broadcast the block. Modifies the template in place,
+     * updating the fields listed below as well as the merkle root.
      *
-     * @returns if the block was processed, independent of block validity
+     * @param[in] version version block header field
+     * @param[in] timestamp time block header field (unix timestamp)
+     * @param[in] nonce nonce block header field
+     * @param[in] coinbase complete coinbase transaction (including witness)
+     *
+     * @note unlike the submitblock RPC, this method does NOT add the
+     *       coinbase witness automatically.
+     *
+     * @returns if the block was processed, does not necessarily indicate validity.
+     *
+     * @note Returns true if the block is already known, which can happen if
+     *       the solved block is constructed and broadcast by multiple nodes
+     *       (e.g. both the miner who constructed the template and the pool).
      */
     virtual bool submitSolution(uint32_t version, uint32_t timestamp, uint32_t nonce, CTransactionRef coinbase) = 0;