Avoid passing size = -1 to PutObject API by requiring content-length
header in POST request (as AWS S3 does) and in Upload web handler.
Post handler is modified to completely store multipart file to know
its size before sending it to PutObject().
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%
```
Anis Elleuch