memdb: fix data race
desc: FileDesc.SetMainManifest: don't remove old file first; overwrite it with new file instead
memdb: fix key/value overwrite in DB.Put; and fix API inconsistency

- rename old DB.Get method to DB.Find
- now DB.Get method will only return exact match
- now DB.Size will return sum of key/value size
- better DB.findLT and DB.findLast; return nil if reaching DB.head
- DB.Put should able to overwrite key/value correctly
Merge branch 'master' of gitlab.svc:glide/goleveldb