While rewinding Chronik or while catching up blocks to the node, the user should be able to cancel the process gracefully, using Ctrl+C for example.

We add ffi bindings to ShutdownRequested and check for it in rewind_indexer when rewinding blocks, in resync_indexer when adding blocks, and in try_setup_chronik to short-circuit any further Chronik setup.

Chronik is way too fast when indexing empty blocks, so a functional test is too difficult/expensive.

1. ninja
2. ./src/bitcoind
3. Let it sync for 10000 blocks, Ctrl+C
4. ./src/bitcoind -chronik -debug=chronik
5. Ctrl+C while Chronik is catching up
6. Chronik re-sync immediately stops, and the node shuts down orderly