An SRV RR contains a priority value for each of the SRV targets that
are present, ranging from 0 - 65535. When caching SRV records we may want to
filter out any targets above or below a particular threshold.
This change adds support for specifying a lower and/or upper bound on
target priorities for any SRV RRs. Any targets returned when resolving
the SRV RR whose priority does not fall between the lower and upper
thresholds are ignored.
For example: Let's say we are running two Redis servers, a primary and
cold server as a backup (but not a replica). Both servers would pass health
checks, but clearly the primary should be preferred over the backup
server. In this case, we could configure our SRV RR with the primary
target as priority 1 and backup target as priority 10. The
`DISCOURSE_REDIS_HOST_SRV_LE` could then be set to 1 and the target with
priority 10 would be ignored.
See /t/66045.
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