distsql: fix wrong schema version when snapshot has been set (#15258)#15872

Merged

sre-bot merged 5 commits intopingcap:release-3.1from sre-bot:release-3.1-1637c42d45db

Mar 31, 2020

+59-2

Commits on Mar 30, 2020

cherry pick pingcap#15258 to release-3.1

lzmhhh123
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sre-bot
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