scripted-diff: wallet: Rename pcoin to wtx
scripted-diff: wallet: Rename pcoin to wtx

Summary:
```
-BEGIN VERIFY SCRIPT-
sed -i --regexp-extended -e 's/const CWalletTx ?\* ?pcoin = &/const CWalletTx\& wtx = /g' src/wallet/wallet.cpp
sed -i -e 's/\<pcoin->/wtx./g' src/wallet/wallet.cpp
sed -i -e 's/\<pcoin\>/\&wtx/g' src/wallet/wallet.cpp
-END VERIFY SCRIPT-
```

This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15596 | PR15596]] : https://github.com/bitcoin/bitcoin/pull/15596/commits/faa3a246e8809b4954a4a6d202fe8c7f7f776b6e

Depends on D6221

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6222

cculianu/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc