refactor: replace RecursiveMutex m_added_nodes_mutex with Mutex
refactor: replace RecursiveMutex m_added_nodes_mutex with Mutex

The RecursiveMutex m_added_nodes_mutex is used at three places:
    - CConnman::GetAddedNodeInfo()
    - CConnman::AddNode()
    - CConnman::ThreadOpenConnections()
In each of the critical sections, only the the m_added_nodes member is
accessed (and in the last case, also m_addr_fetches), without any chance
that within one section another one is called. Hence, we can use an
ordinary Mutex instead of RecursiveMutex.

domob1812/namecore,kallewoof/bitcoin,instagibbs/bitcoin,achow101/bitcoin,fujicoin/fujicoin,sipsorcery/bitcoin,bitcoinknots/bitcoin,sstone/bitcoin,anditto/bitcoin,anditto/bitcoin,instagibbs/bitcoin,GroestlCoin/GroestlCoin,achow101/bitcoin,AkioNak/bitcoin,namecoin/namecoin-core,namecoin/namecoin-core,pataquets/namecoin-core,jamesob/bitcoin,domob1812/bitcoin,Xekyo/bitcoin,GroestlCoin/GroestlCoin,jamesob/bitcoin,ajtowns/bitcoin,ajtowns/bitcoin,jlopp/statoshi,fujicoin/fujicoin,instagibbs/bitcoin,sipsorcery/bitcoin,tecnovert/particl-core,Xekyo/bitcoin,andreaskern/bitcoin,mruddy/bitcoin,domob1812/bitcoin,fanquake/bitcoin,GroestlCoin/GroestlCoin,bitcoinknots/bitcoin,namecoin/namecoin-core,ajtowns/bitcoin,mruddy/bitcoin,GroestlCoin/GroestlCoin,AkioNak/bitcoin,dscotese/bitcoin,fanquake/bitcoin,mm-s/bitcoin,domob1812/namecore,bitcoinknots/bitcoin,particl/particl-core,lateminer/bitcoin,bitcoinsSG/bitcoin,jambolo/bitcoin,namecoin/namecoin-core,kallewoof/bitcoin,kallewoof/bitcoin,sstone/bitcoin,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,dscotese/bitcoin,bitcoin/bitcoin,GroestlCoin/bitcoin,andreaskern/bitcoin,bitcoinsSG/bitcoin,particl/particl-core,jlopp/statoshi,namecoin/namecore,dscotese/bitcoin,pataquets/namecoin-core,fujicoin/fujicoin,GroestlCoin/bitcoin,particl/particl-core,ajtowns/bitcoin,particl/particl-core,mruddy/bitcoin,pataquets/namecoin-core,jambolo/bitcoin,sipsorcery/bitcoin,andreaskern/bitcoin,domob1812/bitcoin,anditto/bitcoin,namecoin/namecoin-core,bitcoinsSG/bitcoin,bitcoin/bitcoin,AkioNak/bitcoin,domob1812/bitcoin,anditto/bitcoin,particl/particl-core,jamesob/bitcoin,AkioNak/bitcoin,fanquake/bitcoin,sstone/bitcoin,kallewoof/bitcoin,pataquets/namecoin-core,mruddy/bitcoin,mruddy/bitcoin,namecoin/namecore,GroestlCoin/bitcoin,fujicoin/fujicoin,instagibbs/bitcoin,domob1812/namecore,instagibbs/bitcoin,sstone/bitcoin,bitcoinsSG/bitcoin,jlopp/statoshi,mm-s/bitcoin,Xekyo/bitcoin,GroestlCoin/bitcoin,lateminer/bitcoin,pataquets/namecoin-core,jambolo/bitcoin,namecoin/namecore,namecoin/namecore,jambolo/bitcoin,mm-s/bitcoin,bitcoin/bitcoin,bitcoinknots/bitcoin,instagibbs/bitcoin,particl/particl-core,mm-s/bitcoin,mruddy/bitcoin,fanquake/bitcoin,bitcoin/bitcoin,namecoin/namecore,fanquake/bitcoin,sipsorcery/bitcoin,dscotese/bitcoin,GroestlCoin/GroestlCoin,tecnovert/particl-core,fujicoin/fujicoin,andreaskern/bitcoin,achow101/bitcoin,jamesob/bitcoin,dscotese/bitcoin,achow101/bitcoin,mm-s/bitcoin,kallewoof/bitcoin,AkioNak/bitcoin,bitcoin/bitcoin,anditto/bitcoin,bitcoinknots/bitcoin,domob1812/namecore,bitcoinsSG/bitcoin,fujicoin/fujicoin,tecnovert/particl-core,tecnovert/particl-core,jambolo/bitcoin,sstone/bitcoin,bitcoin/bitcoin,Xekyo/bitcoin,GroestlCoin/bitcoin,dscotese/bitcoin,bitcoinsSG/bitcoin,jlopp/statoshi,ajtowns/bitcoin,lateminer/bitcoin,lateminer/bitcoin,andreaskern/bitcoin,GroestlCoin/bitcoin,ajtowns/bitcoin,jamesob/bitcoin,achow101/bitcoin,namecoin/namecore,AkioNak/bitcoin,pataquets/namecoin-core,Xekyo/bitcoin,lateminer/bitcoin,domob1812/namecore,andreaskern/bitcoin,kallewoof/bitcoin,jambolo/bitcoin,sstone/bitcoin,anditto/bitcoin,sipsorcery/bitcoin,domob1812/bitcoin,tecnovert/particl-core,jlopp/statoshi,lateminer/bitcoin,namecoin/namecoin-core,achow101/bitcoin,Xekyo/bitcoin,jlopp/statoshi,mm-s/bitcoin,fanquake/bitcoin,jamesob/bitcoin,domob1812/namecore,domob1812/bitcoin,tecnovert/particl-core