fix: healthcheck to targets stopped after update upstream

[locao]

Same as #78, but rebased to release/1.4.x branch.

Original description:

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Hi,

As I reported in the issue Kong/kong#7652

When I updated an upstream, healthcheck to targets of this upstream has been stopped.

The bug is by using weak table and ipairs
So, when I updated an upstream, Kong insert a new object contain upstream information to hcs table
When Lua GC removed old object of the upstream => loop with ipairs stopped when got first nil => any upstream object after nil value not will be checked

-- checker objects (weak) table
local hcs = setmetatable({}, {
    __mode = "v",
})

...

for _, checker_obj in ipairs(hcs) do

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Closes: #78