lightning: ensure the web interface still works outside server mode

[kennytm]

What problem does this PR solve?

Fix #231.

What is changed and how it works?

Implement GET /tasks, GET /tasks/$id and DELETE /tasks/$id even when the config list is nil.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)
  • run Lightning without server mode and access the web interface.

Side effects

Related changes

• Need to be included in the release note

[kennytm]

/run-all-tests

[kennytm]

PTAL @july2993 @amyangfei

[lance6716]

Implement GET /tasks, GET /tasks/$id and DELETE /tasks/$id even when the config list is nil.

"when the config list is nil" happens only in non-server mode? That's not as apparent as "non-server mode"

Rest LGTM

[kennytm]

@lance6716 yeah this is explained in

tidb-lightning/lightning/lightning.go

Lines 258 to 263 in f9f6d8b

	if l.taskCfgs == nil {
	// l.taskCfgs is non-nil only if Lightning is started with RunServer().
	// Without the server mode this pointer is default to be nil.
	writeJSONError(w, http.StatusNotImplemented, "server-mode not enabled", nil)
	return
	}

In non-server mode, each execution only run one task, so there is no need to initialize the config list. This was used to deduce the error message.

[kennytm]

/run-all-tests

[IANTHEREAL]

LGTM