Support failpoints group

[hunterlxt]

Let multiple tests run over different threads, meanwhile each configuration doesn't affect others.

• expose FailPointRegistry as local registry.
• pub fn create_registry() -> FailPointRegistry can get a new FailPointRegistry. So we can bound threads into exact one registry with register_current(&self) and deregister_current(&self).
• add a test to show multiple threads don't affect each other.

close #51

[hunterlxt]

I think all the tests need to be updated, It is better to test in the form of fail_points!. But I'm busy for special week now, and will continue to improve unit testing later.

[hunterlxt]

@BusyJay I think this PR is ready PTAL

[zhangjinpeng87]

@NingLin-P PTAL

[BusyJay]

I suggest to use the patch inside TiKV if the interfaces are good enough to write clean code. As far as what I can see, it's hard to build thread group without a FailPointRegistry::current method.

[hunterlxt]

This PR is staled for about a week

[BusyJay]

So have you tried to use it in TiKV? Is everything working well?

[hunterlxt]

tikv/tikv#8065 @BusyJay