Support UpdateTimestamp log entry property

The UpdateTimestamp property says when an event log property was last
updated.  On log creation, this field is initialized with the time that
the log was created.

The only other time a log entry property can change is when the Resolved
property changes, so also added support there to update the new
UpdateTimestamp property.

While technically every property of the Entry interface is writeable,
there is no use case nor code for changing those in the field, so
UpdateTimestamp does not support those.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I9b1360b9c5424e130947045cee7b64ede6264829
diff --git a/elog_entry.cpp b/elog_entry.cpp
index 04aa917..36a3fb2 100644
--- a/elog_entry.cpp
+++ b/elog_entry.cpp
@@ -25,6 +25,12 @@
         current =
             sdbusplus::xyz::openbmc_project::Logging::server::Entry::resolved(
                 value);
+
+        uint64_t ms = std::chrono::duration_cast<std::chrono::milliseconds>(
+                          std::chrono::system_clock::now().time_since_epoch())
+                          .count();
+        updateTimestamp(ms);
+
         serialize(*this);
     }