Similar to `weldr/json.go`, this file will contain structs that are
shared between client and server and only meant to be used to
serialize from and to JSON.
This immediate change allows us to make fields in `Job` private in a
future commit.
This moves the client code into the same package as the server code,
which makes it easier to change (and version) the two in sync. Also, it
will allow to make some structs private to the jobqueue package and to
test `Client`.
Also rename it to jobqueue.Client.
