Fix the crash when cluster commit version is largely advanced.

[RenxuanW]

1. When backup sees a large version gap, print a SevError and stop;
2. If there are any active backup, prevent a restore.

20210509-023950-renxuan-b5a235b28d943b77 compressed=True data_size=23062715 duration=3728369 ended=101340 fail=1 fail_fast=10 max_runs=100000 pass=100400 priority=100 remaining=0 runtime=0:21:30 sanity=False started=101407 stopped=20210509-030120 submitted=20210509-023950 timeout=5400 username=renxuan

The failed test is fast/LowLatency.toml, unrelated.

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[jzhou77]

@sfc-gh-ljoswiak is working on a fix for this problem, can you please link here?

[sfc-gh-ljoswiak]

#6743 is a WIP PR to fix the restore side. There will be a separate PR to fix backups as well.