feat(protocol): job messages, protocol version and job model (#7)

internal/master/store/migrations/0002_jobs.sql

@@ -0,0 +1,27 @@
+ALTER TABLE services ADD COLUMN config_files text NOT NULL DEFAULT '';
+ALTER TABLE services ADD COLUMN desired_ref  text NOT NULL DEFAULT '';
+
+CREATE TABLE update_jobs (
+    id               bigserial PRIMARY KEY,
+    service_id       bigint NOT NULL REFERENCES services (id),
+    agent_id         bigint REFERENCES agents (id),
+    from_ref         text NOT NULL DEFAULT '',
+    to_ref           text NOT NULL,
+    trigger          text NOT NULL CHECK (trigger IN ('manual', 'webhook', 'poll')),
+    requested_by     text NOT NULL DEFAULT '',
+    status           text NOT NULL CHECK (status IN ('queued', 'running', 'succeeded', 'failed')),
+    output           text NOT NULL DEFAULT '',
+    output_truncated boolean NOT NULL DEFAULT false,
+    error            text NOT NULL DEFAULT '',
+    created_at       timestamptz NOT NULL,
+    started_at       timestamptz,
+    finished_at      timestamptz
+);
+
+-- One queued or running job per service. CreateJob relies on this index to
+-- turn a second concurrent click into ErrJobActive.
+CREATE UNIQUE INDEX update_jobs_one_active ON update_jobs (service_id)
+    WHERE status IN ('queued', 'running');
+CREATE INDEX update_jobs_by_service ON update_jobs (service_id, id DESC);
+CREATE INDEX update_jobs_active_by_agent ON update_jobs (agent_id)
+    WHERE status IN ('queued', 'running');