refactor: revert m_next_resend to not be std::atomic
refactor: revert m_next_resend to not be std::atomic

Since m_next_resend is now only called from MaybeResendWalletTxs()
we don't have any potential race conditions anymore, so the usage
of std::atomic can be reverted.

Github-Pull: #26205
Rebased-From: b01682a812f0841170657708ef0e896b904fcd77

fujicoin/fujicoin,fujicoin/fujicoin,fujicoin/fujicoin,fujicoin/fujicoin,fujicoin/fujicoin,fujicoin/fujicoin