Copy the smart pointer rather then the raw tx when adding a mempool tx to the wallet

Summary: This will allow for making CTransaction non copyable, and force the use of CTransactionRef so we only keep a single pointer to a transaction for all its uses through the codebase.

Test Plan:

ninja all check-all
./test/functional/test_runner.py wallet*
./contrib/teamcity/build-configurations.py build-asan

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12916