ovsdb: Save some space in the log for newly inserted records.

ovsdb: Save some space in the log for newly inserted records.

When a new record is inserted into a database, ovsdb logs the values of all
of the fields in the record.  However, often new records have many columns
that contain default values.  There is no need to log those values, so this
commit causes them to be omitted.

As a side effect, this also makes "ovsdb-tool show-log --more --more"
output easier to read, because record insertions print less noise.  (Adding
--more --more to this command makes it print changes to database records.
The --more option will be introduced in an upcoming commit.)