*: fix update relay processing order

[amyangfei]

What problem does this PR solve?

in DM-worker UpdateRelayConfig function, the check of update source ID is not allowed is before new config loaded, which will always fail.
Besides I find we will get the following error after we update a relay config. This is caused by we forget to close binlog replication syncer after relay paused

2019/05/10 06:19:52.383 relay.go:176: [error] [relay] process exit with error ERROR 1236 (HY000): A slave with the same server_uuid/server_id as this slave has connected to the master; the first event 'bin.000007' at 27622680, the last event read from './bin.000007' at 27622993, the last byte read from './bin.000007' at 27622993.
github.com/pingcap/errors.AddStack
        /root/.gvm/pkgsets/go1.12/global/pkg/mod/github.com/pingcap/errors@v0.11.0/errors.go:174
github.com/pingcap/errors.Trace
        /root/.gvm/pkgsets/go1.12/global/pkg/mod/github.com/pingcap/errors@v0.11.0/juju_adaptor.go:12
github.com/pingcap/dm/relay.(*Relay).process
        /root/code/gopath/src/github.com/pingcap/dm/relay/relay.go:357
github.com/pingcap/dm/relay.(*Relay).Process
        /root/code/gopath/src/github.com/pingcap/dm/relay/relay.go:173
github.com/pingcap/dm/dm/worker.(*RelayHolder).run
        /root/code/gopath/src/github.com/pingcap/dm/dm/worker/relay.go:107
github.com/pingcap/dm/dm/worker.(*RelayHolder).resumeRelay.func1
        /root/code/gopath/src/github.com/pingcap/dm/dm/worker/relay.go:200
runtime.goexit
        /root/.gvm/gos/go1.12/src/runtime/asm_amd64.s:1337
2019/05/10 06:19:52.383 relay.go:113: [error] process error with type UnknownError:

What is changed and how it works?

1. Load the new DM-worker config before any validation
2. close binlog syncer in TCPReader.Close

Check List

Tests

• Integration test
  • more integration test with dmctl API will be added in pr dmctl: use dmctl binary directly in integration test #135
• Manual test (add detailed scripts or steps below)
  • deploy a DM cluster
  • run update-relay -w worker_host:worker_port path_to_new_dm_worker_config in dmctl
  • check update-relay is success

[amyangfei]

/run-all-tests

[amyangfei]

/run-all-tests

[zhouqiang-cl]

/run-all-tests

[zhouqiang-cl]

/run-all-tests

[zhouqiang-cl]

/run-all-tests

[amyangfei]

PTAL @GregoryIan @csuzhangxc

[csuzhangxc]

LGTM

[codecov]

Codecov Report

Merging #142 into master will increase coverage by 0.0766%.
The diff coverage is 12.5%.

@@               Coverage Diff                @@
##             master       #142        +/-   ##
================================================
+ Coverage   29.8727%   29.9493%   +0.0766%     
================================================
  Files           115        115                
  Lines         12647      12638         -9     
================================================
+ Hits           3778       3785         +7     
+ Misses         8436       8420        -16     
  Partials        433        433

[amyangfei]

codecov diff hit means the coverage rate of diff code, it is not stable now, may be we can ignore

[IANTHEREAL]

LGTM

[amyangfei]

/run-all-tests

[amyangfei]

/run-all-tests

[zhouqiang-cl]

/run-all-tests