[backport]Give WalletModel::UnlockContext move semantics
Summary:
WalletModel::UnlockContext seems to implement "move upon copy" semantics; with C++11 this can be done more safely using move semantics (making attempts to actually copy fail instead).
Backport of Core PR16001
Test Plan: ninja check
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7567