* agent: add option to discard health output
In high volatile environments consul will have checks with "noisy"
output which changes every time even though the status does not change.
Since the output is stored in the raft log every health check update
unblocks a blocking call on health checks since the raft index has
changed even though the status of the health checks may not have changed
at all. By discarding the output of the health checks the users can
choose a different tradeoff. Less visibility on why a check failed in
exchange for a reduced change rate on the raft log.
* agent: discard output also when adding a check
* agent: add test for discard check output
* agent: update docs
* go vet
* Adds discard_check_output to reloadable config table.
* Updates the change log.
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