Modify schema to enforce UTC and timezones

[jayjacobs]

Currently the timestamp values in the record are all over the place (see AWG #135)

These are a list of changes the schema to enforce clean storage of time stamps:

• Modify the "timestamp" definition to reference UTC instead of GMT (Use UTC instead of GMT in JSON5 #289)
• Make the inclusion of timezones non-optional (alternatively coordinate with CVE Services to ensure their issue 1274 will handle whatever input is allowed by the schema.
• Remove "datestamp" definition field as it is not referenced or used anywhere in the schema. (the "timestamp" definition is referenced everywhere a date/time object is needed.