Merge bitcoin/bitcoin#13875: [doc] nChainTx needs to become a 64-bit earlier due to SegWit

ef72e9b doc: nChainTx needs to become a 64-bit earlier due to SegWit (Sjors Provoost)

Pull request description:

  As of block 597,379 txcount is 460,596,047 (see `chainparams.cpp`), while `uint32` can handle up to 4,294,967,296.

  Pre segwit the [minimum transaction size](https://en.bitcoin.it/wiki/Maximum_transaction_rate) was 166 bytes, so the worst case number of transactions per block was ~6000. As the original source comment for `unsigned int  nChainTx` says, that should last until the year 2030.

  With SegWit the smallest possible transaction is 60 bytes (potentially increased to 65 with a future soft fork, see #15482), without a witness:

  ```
  4 bytes version
      1 byte input count
          36 bytes outpoint
          1 byte scriptSigLen (0x00)
          0 bytes scriptSig
          4 bytes sequence
      1 byte output count
          8 bytes value
          1 byte scriptPubKeyLen
          1 byte scriptPubKey (OP_TRUE)
      4 bytes locktime
  ```

  That puts the maximum number of transactions per block at 16,666 so we might have to deal with this as early as a block 827,450 in early 2024.

  Given that it's a memory-only thing and we want to allow users many years to upgrade, I would suggest fixing this in v0.20 and back-porting it.

ACKs for top commit:
  practicalswift:
    re-ACK ef72e9b
  jarolrod:
    ACK ef72e9b
  theStack:
    ACK ef72e9b

Tree-SHA512: d8509ba7641796cd82af156354ff3a12ff7ec0f7b11215edff6696e95f8ca0e3596f719f3492ac3acb4b0884ac4e5bddc76f107b656bc2ed95a8ef1b2b5d4f71

Author: laanwj

src/chain.h

@@ -178,7 +178,7 @@ class CBlockIndex
 
     //! (memory only) Number of transactions in the chain up to and including this block.
     //! This value will be non-zero only if and only if transactions for this block and all its parents are available.
-    //! Change to 64-bit type when necessary; won't happen before 2030
+    //! Change to 64-bit type before 2024 (assuming worst case of 60 byte transactions).
     //!
     //! Note: this value is faked during use of a UTXO snapshot because we don't
     //! have the underlying block data available during snapshot load.