Add StorDB service

Add StorDB to config and services.

Put back the store_cdrs option under cdrs and update the CDRs
service to depend on StorDB.

Define the StorDB interface and add a constructor for it.

Add a constructor for postgres storage.

Add a config sanity check to validate SSL modes for postgres.

Update cgr-engine to consider StorDB on startup.

engine/storage_sql.go

@@ -40,6 +40,7 @@ type SQLImpl interface {
 type SQLStorage struct {
 	DB *sql.DB
 	db *gorm.DB
+	StorDB
 	SQLImpl
 }