BTreeMap: add multithreaded test
BTreeMap: add multithreaded test

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