Fix table syncing with different column order

Fix table syncing with different column order

Logical replication supports replicating between tables with different
column order.  But this failed for the initial table sync because of a
logic error in how the column list for the internal COPY command was
composed.  Fix that and also add a test.

Also fix a minor omission in the column name mapping cache.  When
creating the mapping list, it would not skip locally dropped columns.
So if a remote column had the same name as a locally dropped
column (...pg.dropped...), then the expected error would not occur.