Harshavardhana 85f2b74cfd jwt: Cache the bcrypt password hash. (#3526)
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%
```
2017-01-26 16:51:51 -08:00
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