worker: fix potential panic when restoring subtask from meta (#305)

[sre-bot]

cherry-pick #305 to release-1.0

---

What problem does this PR solve?

[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x53b07e]

goroutine 47 [running]:
context.WithDeadline(0x0, 0x0, 0xbf5be448e73dc52d, 0x463dd73766, 0x2ad5a60, 0x0, 0x0, 0x0)
/usr/local/go/src/context/context.go:384 +0x4e
context.WithTimeout(0x0, 0x0, 0x45d964b800, 0xc001974200, 0x2, 0x2)
/usr/local/go/src/context/context.go:451 +0x6b
github.com/pingcap/dm/dm/worker.(*SubTask).unitTransWaitCondition(0xc000372e40, 0x0, 0x0)
/home/jenkins/workspace/build_dm_master/go/src/github.com/pingcap/dm/dm/worker/subtask.go:627 +0x3ca
github.com/pingcap/dm/dm/worker.(*SubTask).run(0xc000372e40)
/home/jenkins/workspace/build_dm_master/go/src/github.com/pingcap/dm/dm/worker/subtask.go:182 +0x5c
github.com/pingcap/dm/dm/worker.(*SubTask).fetchResult(0xc000372e40, 0xc0004a7560)
/home/jenkins/workspace/build_dm_master/go/src/github.com/pingcap/dm/dm/worker/subtask.go:248 +0xb28
created by github.com/pingcap/dm/dm/worker.(*SubTask).run
/home/jenkins/workspace/build_dm_master/go/src/github.com/pingcap/dm/dm/worker/subtask.go:196 +0x4a8

What is changed and how it works?

before this PR, we have the following order in worker.go:

1. InitConditionHub registers Worker instances
2. restoreSubTask restores subtasks from meta
3. w.ctx, w.cancel = context.WithCancel(context.Background()) inits w.ctx and w.cancel

but when restoring subtasks from meta, w.ctx may be used, then nil pointer will be accessed and panicked.

in this PR, we make sure have the following order:

1. w.ctx, w.cancel = context.WithCancel(context.Background()) inits w.ctx and w.cancel
2. InitConditionHub registers Worker instances
3. restoreSubTask restores subtasks from meta

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[amyangfei]

LGTM

[codecov]

Codecov Report

Merging #311 into release-1.0 will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           release-1.0       #311   +/-   ##
==============================================
  Coverage      60.1916%   60.1916%           
==============================================
  Files              134        134           
  Lines            14816      14816           
==============================================
  Hits              8918       8918           
  Misses            5059       5059           
  Partials           839        839

[WangXiangUSTC]

LGTM