sql: Add column job_cancelled

When mxqkill is used against a running job, it has to communicate the
fact, that the job should be aborted to the daemon handling it.

Add a boolean (=TINYINT) column to mxq_job for that.

Other options were considered but discarded, mostly because it would lead to
more complex code:

- Further overload job_status
- Utilize a bit of job_flags
- use of MySQL BIT data type
- use of MySQL SET data type

mysql/create_tables.sql

@@ -72,6 +72,8 @@ CREATE TABLE IF NOT EXISTS mxq_job (
    job_status     INT2 UNSIGNED   NOT NULL DEFAULT 0,
    job_priority   INT2 UNSIGNED   NOT NULL DEFAULT 127,
 
+   job_cancelled  BOOLEAN         NOT NULL DEFAULT FALSE,
+
    group_id       INT8 UNSIGNED   NOT NULL,
 
    job_workdir    VARCHAR(4096)   NOT NULL,

mysql/migrate_015_add_job_cancelled.sql

@@ -0,0 +1,5 @@
+ALTER TABLE mxq_job
+    ADD COLUMN
+        job_cancelled BOOLEAN NOT NULL DEFAULT FALSE
+    AFTER
+        job_priority;