*: unify names of processing units (#456)

[ti-srebot]

cherry-pick #456 to release-2.0

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What is changed, added or deleted? (Required)

Unify the names of processing units in both Chinese and English files, except for those in the v1.0 benchmark reports and 2.0 rc release notes.

Rules for unifying the terms are as follows:
Mydumper/Dumpling processing unit -> dump processing unit/dump unit
Loader processing unit -> load processing unit/load unit
Syncer processing unit -> sync processing unit/sync unit

Names of configuration items and parameters are not included.

Which DM version(s) do your changes apply to? (Required)

• master (the latest development version, including v3.0 changes)
• v2.0 (TiDB DM 2.0 versions)
• v1.0 (TiDB DM 1.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): https://github.com/pingcap/docs-dm/pull/441/files#r511837239, en: update safe mode description #454 (comment)

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Have version specific changes
• Might cause conflicts after cherry-picked