ovsdb: Check for changed columns only once per transaction commit.

ovsdb: Check for changed columns only once per transaction commit.

Until now, each part of a transaction commit that is interested in whether
a column's value has changed has had to do a comparison of the old and new
values itself.  There can be several interested parties per commit
(generally one for file storage and one for each remove OVSDB connection),
so this seems like too much redundancy.  This commit adds a bitmap
to struct ovsdb_txn_row that tracks whether a column's value has actually
changed, to reduce this overhead.

As a convenient side effect of doing these checks up front, it then
becomes easily possible to drop txn_rows (and txn_tables and entire txns)
that become no-ops.  (This probably fixes bug #2400, which reported that
some no-ops actually report updates over monitors.)