ovsdb-idl: Transition to better interfaces for reading table columns.

ovsdb-idl: Transition to better interfaces for reading table columns.

The existing ovsdb_idl_txn_read() was somewhat difficult and expensive to
use, because it always made a copy of the data in the column.  This was
necessary at the time it was introduced, because there was no way for it
to return a "default" value for columns that had not yet been populated
without allocating data and hence requiring the caller to free it.

Now that ovsdb_datum_default() exists, this is no longer required.  This
commit introduces a pair of new functions, ovsdb_idl_read() and
ovsdb_idl_get(), that return a pointer to existing data and do not do any
copying.  It also transitions all of ovsdb_idl_txn_read()'s callers to
the new interfaces.