PLT-6916: Redesign the jobs package and Jobserver. (#6733)

This commit redesigns the jobserver to be based around an architecture
of "workers", which carry out jobs of a particular type, and "jobs"
which are a unit of work carried by a particular worker. It also
introduces "schedulers" which are responsible for scheduling jobs of a
particular type automatically (jobs can also be scheduled manually when
apropriate).

Workers may be run many times, either in instances of the platform
binary, or the standalone jobserver binary. In any mattermost cluster,
only one instance of platform OR jobserver must run the schedulers. At
the moment this is controlled by a config variable, but in future will
be controlled through the cluster leader election process.

model/job.go

@@ -3,7 +3,84 @@
 
 package model
 
-type Job interface {
+import (
+	"encoding/json"
+	"io"
+)
+
+const (
+	JOB_TYPE_DATA_RETENTION  = "data_retention"
+	JOB_TYPE_SEARCH_INDEXING = "search_indexing"
+
+	JOB_STATUS_PENDING          = "pending"
+	JOB_STATUS_IN_PROGRESS      = "in_progress"
+	JOB_STATUS_SUCCESS          = "success"
+	JOB_STATUS_ERROR            = "error"
+	JOB_STATUS_CANCEL_REQUESTED = "cancel_requested"
+	JOB_STATUS_CANCELED         = "canceled"
+)
+
+type Job struct {
+	Id             string                 `json:"id"`
+	Type           string                 `json:"type"`
+	Priority       int64                  `json:"priority"`
+	CreateAt       int64                  `json:"create_at"`
+	StartAt        int64                  `json:"start_at"`
+	LastActivityAt int64                  `json:"last_activity_at"`
+	Status         string                 `json:"status"`
+	Progress       int64                  `json:"progress"`
+	Data           map[string]interface{} `json:"data"`
+}
+
+func (js *Job) ToJson() string {
+	if b, err := json.Marshal(js); err != nil {
+		return ""
+	} else {
+		return string(b)
+	}
+}
+
+func JobFromJson(data io.Reader) *Job {
+	var status Job
+	if err := json.NewDecoder(data).Decode(&status); err == nil {
+		return &status
+	} else {
+		return nil
+	}
+}
+
+func JobsToJson(jobs []*Job) string {
+	if b, err := json.Marshal(jobs); err != nil {
+		return ""
+	} else {
+		return string(b)
+	}
+}
+
+func JobsFromJson(data io.Reader) []*Job {
+	var jobs []*Job
+	if err := json.NewDecoder(data).Decode(&jobs); err == nil {
+		return jobs
+	} else {
+		return nil
+	}
+}
+
+func (js *Job) DataToJson() string {
+	if b, err := json.Marshal(js.Data); err != nil {
+		return ""
+	} else {
+		return string(b)
+	}
+}
+
+type Worker interface {
+	Run()
+	Stop()
+	JobChannel() chan<- Job
+}
+
+type Scheduler interface {
 	Run()
 	Stop()
 }