Add a flag to control whether to backup/restore SEQUENCEs

[kennytm]

Feature Request

Describe your feature request related problem:

After #242, we will unconditionally backup and restore sequences. However, sequence support is still considered somewhat buggy, so we'd like to add a switch to control whether to backup and/or restore sequences.

Problems involving sequence backup & restore:

• The backed up sequence skips all cached values. For example, even if a TiDB server only used up to ID = 10, it may have already allocated up to ID = 1000 in its cache, so the backed up sequence will have ID = 1001. The 900 numbers are wasted. Note that the same behavior will manifest if we use multiple TiDB servers.
• The backed up sequence value does not respect snapshot, so the backed up value may be even higher.
• Cycles are not yet properly supported.

Note that sequences will never "decrease", so there is no risk over-valuating a sequence other than wasting numbers.

Describe the feature you'd like:

1. Add a switch to disable sequence backup (???)
2. Add a switch to disable sequence restore

If a sequence is not restored, tables depending on that sequence will still be successfully restored, but INSERT operations will fail. This is expected behavior.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: