replace io.Discard usage to fix NUMA copy() latencies
On NUMA systems copying from 8K buffer allocated via
io.Discard leads to large latency build-up for every
```
copy(new8kbuf, largebuf)
```
can in-cur upto 1ms worth of latencies on NUMA systems
due to memory sharding across NUMA nodes.
This change adds a `Secret` property to `HelpKV` to identify secrets
like passwords and auth tokens that should not be revealed by the server
in its configuration fetching APIs. Configuration reporting APIs now do
not return secrets.
Harshavardhana
Aditya Manthramurthy