Allow for storage/retrieval of the key in an environment variable

- Generates a key on-demand
- Stores the key's value as a Base64-encoded string in the given environment
  variable
- Requires setEnv which means base 4.7.0
- Uses lookupEnv, as that was available since base 4.6.0 anyway
Add clientsession-generate to generate and print a key

Useful if you want to pre-set the environment variable for all your servers at
once, rather than starting one locally (or not) and copy/pasting the value
printed over to any other servers.