Add support for generated columns

[nicholasd-ff]

Support for sampling tables with generated columns

This change adds support for sampling tables with generated columns
and tests to check the new feature is working correctly.

Fixes #52

Test output:

1..26
 set_config 
------------
 
(1 row)

 setval 
--------
      1
(1 row)

 setval 
--------
      1
(1 row)

ok 1 - child1 should have 100 rows
ok 2 - child2 should have 100 rows
ok 3 - child3 should have 100 rows
ok 4 - child4 should have 100 rows
ok 5 - child5 should have 100 rows
ok 6 - child6 should have 100 rows
ok 7 - child7 should have 100 rows
ok 8 - child8 should have 100 rows
ok 9 - child9 should have 100 rows
ok 10 - child10 should have 100 rows
ok 11 - all parent rows should be fetched due to FKs
ok 12 - escaping
ok 13 - ordered test case broken, this should return by clustered order
ok 14 - long table name should have 10 rows
ok 15 - some_numbers table should have 10 rows
ok 16 - The double_val column is not 2x the base_val
ok 17 - The double_val column is not 2x the base_val
ok 18 - The double_val column is not 2x the base_val
ok 19 - The double_val column is not 2x the base_val
ok 20 - The double_val column is not 2x the base_val
ok 21 - The double_val column is not 2x the base_val
ok 22 - The double_val column is not 2x the base_val
ok 23 - The double_val column is not 2x the base_val
ok 24 - The double_val column is not 2x the base_val
ok 25 - The double_val column is not 2x the base_val
 set_config 
------------
 
(1 row)

 setval 
--------
      1
(1 row)

 setval 
--------
      1
(1 row)

ok 26 - results should be ordered

[nicholasd-ff]

I need to fix a problem with the FK constraints, nearly got it but it means some changes elsewhere to push the columns into the table object.

[nicholasd-ff]

I think I've fixed it, but I'm testing it at an approximately 30% sample rate on our 120GB staging database as a torture test and I'll update the PR if it's successful.

[mla]

Looks great to me, @nicholasd-ff. Changes merged. Thank you!