Added export_csv_all command
Refactoring and optimizations

Now uses common table definition for all operations.
Added index when creatign the 'files' table, in hopes that SELECT's will
be faster (though it does not seem to be a significant improvement so
far).
Added benchmarking for the slow 'SELECT' statement.
Warning if path could not be deleted.
Added diff_sqlite3 command.

Improved performance of sqlite3-based merge method, by adding an index
for the 'files' table, and by querying db_in_large in order to check for
duplicates, instead of reading db_out.