[Chronik] Make ffi::CBlockIndex Sync

Summary:
&CBlockIndex can be shared between threads safely, so it is sound to make CBlockIndex implement Sync.

Currently async code using CBlockIndex can't hold a & reference between await points, but actually this is fine, so we allow it.

There's no change in behavior.

Test Plan: cargo check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16747