Add support for resetting autoincrement sequences

[timgraham]

When loading fixtures that have primary key values of autoincrement columns specified, Snowflake doesn't automatically update the sequence to the next available value. For example, after loading fixtures with pk=1 and pk=2 into a new table, the next object will be created with pk=1 (duplicating an existing value) instead of pk=3. PostgreSQL has the same issue and Django's DatabaseOperations.sequence_reset_sql() method is a hook to generate SQL that updates the next value of the sequence for each table:

SELECT setval(pg_get_serial_sequence(<table>, <column>), coalesce(max(<column>)), 1), max(column) IS NOT null)
FROM <table>

Since Snowflake's ALTER SEQUENCE doesn't provide a way to set the next value of the sequence, I'm not sure if this is feasible.