set suitable default value for process, source and task config items

[csuzhangxc]

Bug Report

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1. What did you do? If possible, provide a recipe for reproducing the error.

2. What did you expect to see?

3. What did you see instead?

we do not set a suitable value for some config items (like statement-size, this may dump too large txn to make TiDB OOM)

4. Versions of the cluster

   • DM version (run dmctl -V or dm-worker -V or dm-master -V):

     DM v2.0.0-rc.2
     

[csuzhangxc]

dumpling has set the default value of statement-size in pingcap/dumpling#160

[lance6716]

some default config which maybe dangerous

use dumpling default value:

• if mydumper in config is not null but lack some items:
  • FileSize is UnspecifiedSize, but seems one big file could have multiple insert statements, will check if it's fine
  • Rows is UnspecifiedSize, don't split table every Rows into multiple files, seems fine because we didn't set it before
  • SessionParams is empty, maybe we could make use of it in future
  • DumpEmptyDatabase, NoViews

overwrite dumpling to DM specified default:

• EscapeBackslash is true (what's this)

task config:

• CleanDumpFile is true

source config:

• didn't call Verify