Initialize database before checking changed txindex

In case no database exists yet, and -txindex(=1) is passed, we currently first check whether fTxIndex differs from -txindex (and ask the user to reindex in that case), and only afterwards initialize the database. By swapping these
around (the initialization is a no-op in case the database already exists), we allow it to be born in txindex mode, without warning.

That also means we don't need to check -reindex anymore, as the wiping/reinit
of the databases happens before checking.

Closes #2782.

Tested all 8 combinations of the database already existing or not, -reindex passed or not, -txindex set or not.