Add ObservedTimestamp Field

[djaglowski]

The notion of an Observed Timestamp is a recent addition to the Logs Data Model.

This field should be added to the library and populated automatically as appropriate.

• Add ObservedTimestamp field to entry.Entry
• entry.New should initialize ObservedTimestamp, but not Timestamp
• All usages of Timestamp must be evaluated. (i.e. what happens if not set?)
• Ensure all preexisting assignments to Timestamp are still valid (excepting entry.New initialization)