Previously we required matching mtimes but that's actually
unnecessary when the question is about whether to skip the
download. We will still update the file's metadata.
Also, adjust behavior when the checksum is weak (Adler32):
in these cases we still depend on equal mtimes.
This gets rid of the csync_statedb sqlite layer and use
the same code and same connection as the rest of the SyncEngine.
Missing functions are added to SyncJournalDb and change a few minor
things (like changing SyncJournalFileRecord::_modtime to be an int64
instead of a QDateTime, like it was in csync).
This will allow us to unify data structures between csync and libsync.
Utility functions like csync_time and c_std are still compiled as C
since we won't need to be coupled with Qt in the short term.
