rpcwallet: Replace pwallet-> with wallet.

Summary:

rpcwallet: Replace boost::optional<T>::emplace with simple assignment of T{}

Optional::emplace() was only added in boost 1.56, see
https://github.com/boostorg/optional/commit/2e583aaf30964afd66c1284aa71d0d8b42fa440d

To simply work around https://github.com/bitcoin/bitcoin/issues/18943,
replace it with assignment of T{}

https://github.com/bitcoin/bitcoin/pull/18946/commits/fa1f840596554ed264d11ee3b3643bf99eb57eb5
Backport note: Bitcoin ABC already enforces boost > 1.59, and uses std::optional anyway, so this first commit
is included only for future backporting convenience.

rpcwallet: Replace pwallet-> with wallet.

pwallet is never null everywhere where it is dereferenced, so simply
replace it with a reference, which can not be null by definition.

https://github.com/bitcoin/bitcoin/pull/18946/commits/fa1f840596554ed264d11ee3b3643bf99eb57eb5

This is a backport of Core PR18946

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9112