example HTTP webserver
environment variables:
HTTP_PORT: port the web server listens on, default: 8080
NUM_WORKERS: number of workers hashing password, default: 100
BUFFER_SIZE: job channel buffer size, default: 1000
MAX_PWD_LENGTH: max password length, default: 64
use the following command to run the webserver with the default values: go run webserver.go
unit test: go test ./...