Revert "catalog: render µs in SQL methods" partially and add new datetime(N) sqlvar type

Changing 'datetime' to render µs can break existing code if the column has
lower precision. For example:

> insert into t values('2015-05-26 12:18:37'); select * from t where x='2015-05-26 12:18:37';
Query OK, 1 row affected (0.00 sec)

+---------------------+
| x                   |
+---------------------+
| 2015-05-26 12:18:37 |
+---------------------+
1 row in set (0.00 sec)

> insert into t values('2015-05-26 12:18:37.2'); select * from t where x='2015-05-26 12:18:37.2';
Query OK, 1 row affected (0.00 sec)

Empty set (0.00 sec)

Another example is when searching for stricly older documents than another one.
The latter could be wrongly included in the results.

Commit e0388ecc
("catalog: render µs in SQL methods") was also incomplete:
there's still no µs support in ZSQLCatalog.

product/ERP5Type/patches/sqlvar.py

@@ -58,14 +58,16 @@ def SQLVar_render(self, md):
             return repr(float(v))
         except Exception:
             t = 'floating-point'
-    elif t=='datetime':
+    elif t.startswith('datetime'):
+        # For subsecond precision, use 'datetime(N)' MySQL type,
+        # where N is the number of digits after the decimal point.
+        n = 0 if t == 'datetime' else int(t[9])
         try:
             v = (v if isinstance(v, DateTime) else DateTime(v)).toZone('UTC')
-            # For subsecond precision in MySQL, use 'datetime(N)' type,
-            # where N is the number of digits after the decimal point.
-            return "'%s.%06u'" % (v.ISO(), v.micros() % 1000000)
+            return "'%s%s'" % (v.ISO(),
+                ('.%06u' % (v.micros() % 1000000))[:1+n] if n else '')
         except Exception:
-            pass
+            t = 'datetime'
     elif t=='nb' and not v:
         t = 'empty string'
     else:
@@ -79,7 +81,10 @@ def SQLVar_render(self, md):
                      % (t, self.__name__, v))
 
 # Patched by yo. datetime is added.
-valid_type=('int', 'float', 'string', 'nb', 'datetime').__contains__
+
+valid_type = 'int', 'float', 'string', 'nb', 'datetime'
+valid_type += tuple(map('datetime(%s)'.__mod__, xrange(7)))
+valid_type = valid_type.__contains__
 
 SQLVar.render = SQLVar_render
 SQLVar.__call__ = SQLVar_render