refactor: replace RecursiveMutex `m_most_recent_block_mutex` with Mutex
refactor: replace RecursiveMutex `m_most_recent_block_mutex` with Mutex

In each of the critical sections, only the the guarded variables are
accessed, without any chance that within one section another one is
called.  Hence, we can use an ordinary Mutex instead of RecursiveMutex.

fanquake/bitcoin,fanquake/bitcoin,anditto/bitcoin,instagibbs/bitcoin,jambolo/bitcoin,fujicoin/fujicoin,anditto/bitcoin,andreaskern/bitcoin,particl/particl-core,sipsorcery/bitcoin,namecoin/namecore,bitcoinsSG/bitcoin,bitcoinsSG/bitcoin,namecoin/namecore,jambolo/bitcoin,lateminer/bitcoin,lateminer/bitcoin,namecoin/namecore,GroestlCoin/bitcoin,andreaskern/bitcoin,sstone/bitcoin,bitcoin/bitcoin,sipsorcery/bitcoin,ajtowns/bitcoin,instagibbs/bitcoin,ajtowns/bitcoin,Xekyo/bitcoin,GroestlCoin/GroestlCoin,jambolo/bitcoin,fanquake/bitcoin,fujicoin/fujicoin,jamesob/bitcoin,ajtowns/bitcoin,kallewoof/bitcoin,bitcoin/bitcoin,AkioNak/bitcoin,fujicoin/fujicoin,namecoin/namecoin-core,GroestlCoin/GroestlCoin,mruddy/bitcoin,kallewoof/bitcoin,namecoin/namecoin-core,domob1812/bitcoin,jamesob/bitcoin,instagibbs/bitcoin,tecnovert/particl-core,particl/particl-core,AkioNak/bitcoin,anditto/bitcoin,namecoin/namecore,AkioNak/bitcoin,namecoin/namecoin-core,fujicoin/fujicoin,AkioNak/bitcoin,fujicoin/fujicoin,GroestlCoin/bitcoin,fanquake/bitcoin,sipsorcery/bitcoin,Xekyo/bitcoin,Xekyo/bitcoin,Xekyo/bitcoin,instagibbs/bitcoin,GroestlCoin/GroestlCoin,fanquake/bitcoin,sstone/bitcoin,Xekyo/bitcoin,Xekyo/bitcoin,andreaskern/bitcoin,GroestlCoin/bitcoin,domob1812/bitcoin,jambolo/bitcoin,GroestlCoin/bitcoin,GroestlCoin/GroestlCoin,tecnovert/particl-core,jamesob/bitcoin,kallewoof/bitcoin,AkioNak/bitcoin,tecnovert/particl-core,GroestlCoin/GroestlCoin,bitcoin/bitcoin,lateminer/bitcoin,particl/particl-core,mruddy/bitcoin,mruddy/bitcoin,sstone/bitcoin,domob1812/bitcoin,lateminer/bitcoin,tecnovert/particl-core,ajtowns/bitcoin,sstone/bitcoin,namecoin/namecoin-core,sipsorcery/bitcoin,andreaskern/bitcoin,namecoin/namecore,tecnovert/particl-core,bitcoinsSG/bitcoin,domob1812/bitcoin,bitcoin/bitcoin,sipsorcery/bitcoin,jamesob/bitcoin,anditto/bitcoin,domob1812/bitcoin,lateminer/bitcoin,sstone/bitcoin,jambolo/bitcoin,mruddy/bitcoin,instagibbs/bitcoin,kallewoof/bitcoin,anditto/bitcoin,ajtowns/bitcoin,instagibbs/bitcoin,bitcoinsSG/bitcoin,jambolo/bitcoin,mruddy/bitcoin,AkioNak/bitcoin,namecoin/namecoin-core,lateminer/bitcoin,namecoin/namecoin-core,particl/particl-core,bitcoin/bitcoin,bitcoin/bitcoin,particl/particl-core,jamesob/bitcoin,jamesob/bitcoin,ajtowns/bitcoin,fujicoin/fujicoin,particl/particl-core,GroestlCoin/bitcoin,andreaskern/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,mruddy/bitcoin,namecoin/namecore,domob1812/bitcoin,sipsorcery/bitcoin,bitcoinsSG/bitcoin,GroestlCoin/bitcoin,sstone/bitcoin,tecnovert/particl-core,anditto/bitcoin,GroestlCoin/GroestlCoin,kallewoof/bitcoin,andreaskern/bitcoin,bitcoinsSG/bitcoin