Refactoring, created new class DbSqlite3.

The DbSqlite3 class abstracts the most common operations on SQLite3
databases for FilePile.
Added store_update command.

The store_update command will use the FilePile's metadata DB to check if
a file has been stored before, by looking it up using it's metadata
(minus sha256). If evetyrhing (including mtime) is unchanged, then
assume that the sha256 is still valid.