ovsdb-idl: Make it possible to omit or pay less attention to columns.

ovsdb-idl: Make it possible to omit or pay less attention to columns.

ovs-vswitchd has no need to replicate some parts of the database.  In
particular, it doesn't need to replicate the bits that it never reads,
such as the external_ids column in the Open_vSwitch table.  This saves
some memory, CPU time, and bandwidth to the database.

Another type of column that benefits from special treatment is "write-only
columns", that is, those that ovs-vswitchd writes and keeps up-to-date but
never expects another client to write, such as the cur_cfg column in the
Open_vSwitch table.  If the IDL reports that the database has changed when
ovs-vswitchd updates such a column, then ovs-vswitchd reconfigures itself
for no reason, wasting CPU time.  This commit also adds support for such
columns.