[IMP] base_import: performances of parse_preview by way of _try_match_date_time
[IMP] base_import: performances of parse_preview by way of _try_match_date_time

* reduce number of patterns: using localised month names is useless
  since we're not setting the locale so it always uses the server's own
* avoid going through the entire strptime machinery: lift the bits of
  TimeRE we're interested in to get regex bits out of strptime patterns
  and just to an re.match to check whether value & pattern match

Further possible optimisation: cache REs and only compile user-provided
patterns dynamically.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo