backport - revert backup compressor from pargzip to pgzip

[tanjinx]

Description

similar as #11
the default pargzip compressor does not work well for us, revert to pgzip.

wget https://github.com/slackhq/vitess/pull/11.patch
git apply -v --include go/vt/mysqlctl/builtinbackupengine.go ~/11.patch
git apply -v --include go/vt/mysqlctl/xtrabackupengine.go ~/11.patch
git apply -v --include test/templates/cluster_endtoend_test.tpl ~/11.patch

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes