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.

sstone/bitcoin,bitcoin/bitcoin,jambolo/bitcoin,kallewoof/bitcoin,particl/particl-core,bitcoin/bitcoin,tecnovert/particl-core,kallewoof/bitcoin,jambolo/bitcoin,bitcoinsSG/bitcoin,kallewoof/bitcoin,sipsorcery/bitcoin,bitcoin/bitcoin,sipsorcery/bitcoin,lateminer/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,lateminer/bitcoin,namecoin/namecoin-core,AkioNak/bitcoin,mruddy/bitcoin,AkioNak/bitcoin,tecnovert/particl-core,particl/particl-core,bitcoinsSG/bitcoin,jambolo/bitcoin,lateminer/bitcoin,jambolo/bitcoin,fanquake/bitcoin,fanquake/bitcoin,sstone/bitcoin,namecoin/namecoin-core,bitcoin/bitcoin,sstone/bitcoin,mruddy/bitcoin,bitcoin/bitcoin,mruddy/bitcoin,tecnovert/particl-core,fanquake/bitcoin,kallewoof/bitcoin,jambolo/bitcoin,sstone/bitcoin,particl/particl-core,tecnovert/particl-core,lateminer/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,AkioNak/bitcoin,kallewoof/bitcoin,sstone/bitcoin,AkioNak/bitcoin,fanquake/bitcoin,lateminer/bitcoin,AkioNak/bitcoin,bitcoinsSG/bitcoin,namecoin/namecoin-core,bitcoinsSG/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,particl/particl-core,jambolo/bitcoin,tecnovert/particl-core,sipsorcery/bitcoin,lateminer/bitcoin,AkioNak/bitcoin,mruddy/bitcoin,tecnovert/particl-core,mruddy/bitcoin,sipsorcery/bitcoin,sipsorcery/bitcoin,sstone/bitcoin,fanquake/bitcoin,bitcoin/bitcoin,namecoin/namecoin-core,namecoin/namecoin-core,sipsorcery/bitcoin,mruddy/bitcoin,namecoin/namecoin-core