Creds don't require secretKeyHash to be calculated
everytime, cache it instead and re-use.
This is an optimization for bcrypt.
Relevant results from the benchmark done locally, negative
value means improvement in this scenario.
```
benchmark old ns/op new ns/op delta
BenchmarkAuthenticateNode-4 160590992 80125647 -50.11%
BenchmarkAuthenticateWeb-4 160556692 80432144 -49.90%
benchmark old allocs new allocs delta
BenchmarkAuthenticateNode-4 87 75 -13.79%
BenchmarkAuthenticateWeb-4 87 75 -13.79%
benchmark old bytes new bytes delta
BenchmarkAuthenticateNode-4 15222 9785 -35.72%
BenchmarkAuthenticateWeb-4 15222 9785 -35.72%
```
This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.
This also fixes `goconst ./...` reported issues.
poornas