postgres driver: errors when timestamp column uses fractions of seconds

[wonder-sk]

When using a column of timestamp without time zone type and the date/time values have higher precision than seconds, things do not work correctly:

• dump of table data will create a diff with values truncated to whole seconds (see allColumnNames() in postgresdriver.cpp)
• if such dump is applied to a different schema and then the two schemas are compared, geodiff will emit update entries (because the original value and new value are not the same), but the entries will not contain any actual changes (only primary key column is listed in the entry) - I would consider such diff invalid
• if a diff from the previous point gets applied, that operation fails (Incorrect SQL query when there are no updates in the PostgreSQL table #181)

Ideally we would want to preserve full original values if possible. And if we need to modify how date/time values are stored in diffs, we should verify the change is backwards compatible.