This makes the queue more type safe and allows to get rid of the
`pendingChannel` and `pendingChannels` helpers, which only existed to
create not-yet-existing pending channels.
The enum is redundant information that can be deduced from the job's
times: queuedAt, startedAt, and finishedAt. Not having it reduces the
potential for inconsistent state.
Now that the "old" `jobqueue` package was renamed to `worker`, add a new
package that contains an interface to an actual job queue. Also add two
implementations: fsjobqueue, a job queue backed by the file system, and
testjobqueue, which can be used as a mock implementation for testing.
These packages are not yet used.
