sql: Add job_cancelled column

When mxqkill is used against a running job, it needs to communicate the
fact that the job should be aborted to the daemon handling it. This
commit adds a boolean (TINYINT) column `job_cancelled` to `mxq_job` for
this purpose.

Other options such as overloading `job_status`, utilizing a bit of
job_flags, and using MySQL BIT or SET data types were considered but
discarded to avoid code complexity.

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;