Fix #157, BTreeMap not thread safe under some conditions
Fix #157, BTreeMap not thread safe under some conditions

ffjiao/mapdb,pettermahlen/mapdb,cocosli/mapdb,sumit784/mapdb,olupas/MapDB,jankotek/MapDB,mebigfatguy/mapdb,hangum/mapdb,morfeo8marc/mapdb,jankotek/mapdb,ringoluo/mapdb,lamfire/mapdb,SleimanJneidi/mapdb,binarytemple/mapdb,olupas/MapDB,jankotek/mapdb,codeaudit/mapdb,mebigfatguy/mapdb,jankotek/MapDB