BTreeMap: fix #209, put operation was not thread safe.
BTreeMap: fix #209, put operation was not thread safe.

Solved by fixing three TODOs (root locking, tree levels and left most node jump)

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