ZMySQLDA: Optimise date & datetime parsing in results.

A pprofile run shows DateTime_or_None costing 3ms per value (including all
callees).
While this may seem low, it is common to receive hundreds of such values
in single query results (ex: web mode document & webpage lookups),
so the total time spent parting dates per query can become significantly
larger than the time to execute the query itself.
In the case of this measure setup, 504 dates were processed for a total of
1.5 seconds.
DateTime is slow to parse strings because it cannot rely on a pre-known
format. With MySQL/MariaDB, the format is know for those column types, so
implement specific parsing to get better performance.
On the same setup, pprofile run shows a parsing time of 72µs per value in
DATETIME_to_DateTime_or_None, or 39ms total.
DATE_to_DateTime_or_None should not deviate significantly from this.

product/ZMySQLDA/db.py

@@ -158,9 +158,39 @@ def _mysql_timestamp_converter(s):
                           s[8:10],s[10:12],s[12:14]))
         return DateTime("%04d-%02d-%02d %02d:%02d:%02d" % tuple(parts))
 
-def DateTime_or_None(s):
-    try: return DateTime('%s UTC' % s)
-    except Exception: return None
+# DateTime(str) is slow. As the date format is part of the specifications,
+# parse it ourselves to save time.
+def DATETIME_to_DateTime_or_None(s):
+    try:
+        date, time = s.split(' ')
+        year, month, day = date.split('-')
+        hour, minte, second = time.split(':')
+        return DateTime(
+            int(year),
+            int(month),
+            int(day),
+            int(hour),
+            int(minute),
+            float(second),
+            'UTC',
+        )
+    except Exception:
+        return None
+
+def DATE_to_DateTime_or_None(s):
+    try:
+        year, month, day = s.split('-')
+        return DateTime(
+            int(year),
+            int(month),
+            int(day),
+            0,
+            0,
+            0,
+            'UTC',
+        )
+    except Exception:
+        return None
 
 def ord_or_None(s):
     if s is not None:
@@ -171,8 +201,8 @@ class DB(TM):
 
     conv=conversions.copy()
     conv[FIELD_TYPE.LONG] = int
-    conv[FIELD_TYPE.DATETIME] = DateTime_or_None
-    conv[FIELD_TYPE.DATE] = DateTime_or_None
+    conv[FIELD_TYPE.DATETIME] = DATETIME_to_DateTime_or_None
+    conv[FIELD_TYPE.DATE] = DATE_to_DateTime_or_None
     conv[FIELD_TYPE.DECIMAL] = float
     conv[FIELD_TYPE.BIT] = ord_or_None
     del conv[FIELD_TYPE.TIME]