jobqueue: introduce the concept of channels

Channels are a concept similar to job types. Callers must specify a channel
name when queueing a new job. A list of channels is also specified when
dequeueing a job. The dequeued job's channel will always be from one of the
specified channel. Of course, the job types are also respected. The dequeued
job will also always be from one of the specified type.

Currently, all calls to jobqueue were changed so all queue operations use
an empty channel name and all dequeue operations use a list containing
an empty channel.

Thus, this is a non-functional change.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/jobqueue/dbjobqueue/schemas/002_channels.sql

@@ -0,0 +1,15 @@
+ALTER TABLE jobs
+ADD COLUMN channel varchar NOT NULL DEFAULT '';
+
+-- We added a column, thus we have to recreate the view.
+CREATE OR REPLACE VIEW ready_jobs AS
+SELECT *
+FROM jobs
+WHERE started_at IS NULL
+  AND canceled = FALSE
+  AND id NOT IN (
+    SELECT job_id
+    FROM job_dependencies JOIN jobs ON dependency_id = id
+    WHERE finished_at IS NULL
+)
+ORDER BY queued_at ASC