Metrics and logs - MongoDB Submission (#43)

Adds MongoDB-backed submission for survey statistics/metrics and introduces MongoDB ObjectID-based participant/task identifiers to support upserts and linking metrics to a participant.

It introduces breaking changes to the exsisting json stat and metric files. Delete the old ones

Changes:

Add internal/storage/MongoStorage and wire pm survey submit / pm start to submit local .pm/*.json files to MongoDB.
Extend stats/metrics models to include MongoDB _id / participant_id fields and add participant ID propagation into task metrics.
Update app initialization to generate ObjectIDs for new participants and expose GetParticipantID() via StatsService.

internal/commands/survey/start.go

@@ -7,6 +7,7 @@ import (
 )
 
 var (
+	DevFlag       bool
 	InterfaceType string
 	Task          string
 )
@@ -22,4 +23,5 @@ func init() {
 	StartCmd.Flags().StringVarP(&InterfaceType, "interface", "i", "", "Specify interface.")
 	StartCmd.RegisterFlagCompletionFunc("task", shellcomp.CompletionFunc(task.ListTasks()))
 	StartCmd.RegisterFlagCompletionFunc("interface", shellcomp.CompletionFunc(task.ListInterfaces()))
+	StartCmd.Flags().BoolVar(&DevFlag, "dev", false, "Enable development mode, which skips database connection, submission and intro")
 }