Add the global failpoint lock pattern directly to the library

[brson]

When running failpoint unit tests, one must take a global lock so the failpoint configuration stays consistent during parallel execution. We do this in our own failpoints tests, and it's explained extensively in the fail docs. Since the library is significantly less useful without a global lock we might one directly to the library and use them in the tikv failpoints test.

Just copy the pattern from tikv/tests/failpoints into this library, then test tikv against the new failpoints library. This can be done by temporarily replacing the fail dependency in Cargo.toml with a path dependency to the modified version of fail, then running cargo test --test failpoints.

If it all works, then submit the patch here.