Adding table to track workflow executions.

- Data from this table will be used to generate workflow statistics.

db/20200413141037_createworkflowexecutionstatistics.sql

@@ -0,0 +1,20 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE workflow_executions
+(
+    id                   serial PRIMARY KEY,
+    workflow_template_id integer     NOT NULL REFERENCES workflow_templates ON DELETE CASCADE,
+    name                 text        NOT NULL CHECK (name <> ''),
+    namespace            varchar(36) NOT NULL,
+
+    -- auditing info
+    created_at           timestamp   NOT NULL DEFAULT (NOW() at time zone 'utc'),
+    finished_at          timestamp            DEFAULT NULL,
+    failed_at            timestamp            DEFAULT NULL
+);
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+DROP TABLE workflow_executions;
+-- +goose StatementEnd