secondary_mode doesn't work as described in the document

[dandinw]

this is configuration of parent.config:

dest_domain=www.test2.com scheme=http parent="192.168.2.17:10000;192.168.2.17:82" secondary_parent="192.168.2.17:100001" round_robin=consistent_hash secondary_mode=2

I want ATS to try all the parent nodes in the parent list first. When the parent node in the parent list fails or marked down , try the parent node in the second_parent list，

Now the situation is，192.168.2.17:10000 and 192.168.2.17:100001 is an unavailable port，When 192.168.2.17:10000 is unavailable, ATS goes to 192.168.2.17:100001，It ignores 192.168.2.17:82

here is the diag.logs:
[Jul 21 13:56:52.139] [ET_TASK 0] NOTE: parent.config loading ...
[Jul 21 13:56:52.141] [ET_TASK 0] NOTE: parent.config finished loading
[Jul 21 13:56:54.829] [ET_NET 0] NOTE: Parent initially marked as down 192.168.2.17:10000
[Jul 21 13:56:54.849] [ET_NET 0] NOTE: Parent initially marked as down 192.168.2.17:100001

@calavera @janfrode @biilmann @jpeach @sekimura