sql.py: put_table(): DuplicateKeyException: Assert that join_cols has changed to avoid infinite loops

sql.py: put_table(): Moved getting pkeys of already existing rows from DuplicateKeyException to try clause, so that it always runs if there are join_cols. DuplicateKeyException: Add new duplicate key cols to join_cols instead of replacing join_cols so that multiple unique constraints being violated causes the union of their columns to be used for join_cols.

sql_gen.py: Added CustomCode

sql.py: mk_select(): joins: Fixed bug where joins dict was being modified without first being copied, causing the input value to be modified

Compare object()-based sentinel values using is. Where sentinel values must be compared using ==, use rand.rand_int() instead.

sql.py: put_table(): Added debug messages for every action performed

sql.py: put_table(): Moved assignment of in_pkeys_ref outside loop so it wouldn't need to be re-versioned every iteration

sql.py: put_table(): Changed temp_suffix to temp_prefix so all temp tables for a given out_table would have the same prefix. (Existing name collisions due to truncated names are not a problem because version prefixes are automatically added.)

mappings/DwC2-VegBIEN.specimens.csv: Filter dates through _toTimestamp

schemas/functions.sql: Added _toTimestamp