This adds three features flags: SHA1CSRs, OldTLSOutbound, and
OldTLSInbound. Each controls the behavior of an upcoming deprecation
(except OldTLSInbound, which isn't yet scheduled for a deprecation
but will be soon). Note that these feature flags take advantage of
`features`' default values, so they can default to "true" (that is, each
of these features is enabled by default), and we set them to "false"
in the config JSON to turn them off when the time comes.
The unittest for OldTLSOutbound requires that `example.com` resolves
to 127.0.0.1. This is because there's logic in the VA that checks
that redirected-to hosts end in an IANA TLD. The unittest relies on
redirecting, and we can't use e.g. `localhost` in it because of that
TLD check, so we use example.com.
Fixes#6036 and #6037
cf9df961ba