[wallet] Construct CWalletTx objects in CommitTransaction

Summary:
Construct CWalletTx objects in CWallet::CommitTransaction, instead of
having callers do it. This ensures CWalletTx objects are constructed in
a uniform way and all fields are set.

This also makes it possible to avoid confusing and wasteful CWalletTx
copies in https://github.com/bitcoin/bitcoin/pull/9381

There is no change in behavior.

Partial backport of core PR9680 (commit a128bdc)
Depends on D2611

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2618