A ton of style change to wallet.cpp

Summary:

• Remove blanket std import from wallet.cpp
• Add braces around control flow statements
• Remove useless scopes.
• Use nullptr instead of NULL
• Various other small changes.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, sickpig, kyuupichan, awemany, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D301