ovsdb-idl: Fix atomicity of writes that don't change a column's value.

ovsdb-idl: Fix atomicity of writes that don't change a column's value.

The existing ovsdb_idl_txn_commit() drops any writes that don't change a
column's value from what was last reported by the database.  But this isn't
a valid optimization, because it breaks the atomicity of transactions.
Suppose columns A and B initially have values 1 and 2.  Client 1 writes
value 1 to both columns in one transaction.  Client 2 writes value 2 to
both columns in another transaction.  The only possible valid results for
any serial ordering of transactions are 1,1 or 2,2.  But if both clients
drop writes to columns that they have not modified, then 2,1 also becomes
possible (because client 1 just writes to B and client 2 just writes to A).

However, for write-only columns we can optimize this out because the IDL
can assume it is the only client writing to a column.

Found by inspection.