TS-4747: make marking parent proxies down in hostdb configurable.

[jrushford]

Back port of #1464 to 6.2.2

(cherry picked from commit 2752c75)

Conflicts:
doc/admin-guide/files/records.config.en.rst
doc/developer-guide/api/functions/TSHttpOverridableConfig.en.rst
lib/ts/apidefs.h.in
mgmt/RecordsConfig.cc
plugins/experimental/ts_lua/ts_lua_http_config.c
proxy/InkAPI.cc
proxy/InkAPITest.cc
proxy/http/HttpConfig.cc
proxy/http/HttpConfig.h

[atsci]

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[atsci]

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[zwoop]

Set a milestone please. It'd also be nice to have a reference to the original PR #.

[PSUdaemon]

Can you rebase this so we can get the Intel build clear? Thanks.

[jrushford]

[approve ci]

[atsci]

Linux build successful! See https://ci.trafficserver.apache.org/job/linux-github/1572/ for details.

[atsci]

FreeBSD build successful! See https://ci.trafficserver.apache.org/job/freebsd-github/1676/ for details.

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Intel CC build successful! See https://ci.trafficserver.apache.org/job/icc-github/109/ for details.

[atsci]

Linux build successful! See https://ci.trafficserver.apache.org/job/linux-github/1573/ for details.

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