* `bdk_chain` dependency is added. In the future, we will introduce a
separate `bdk_core` crate to contain shared types.
* replace `Emitter::new` with `from_height` and `from_checkpoint`
* `from_height` emits from the given start height
* `from_checkpoint` uses the provided cp to find agreement point
* introduce logic that ensures emitted blocks can connect with
receiver's `LocalChain`
* in our rpc example, we can now `expect()` chain updates to always
since we are using checkpoints and receiving blocks in order
Instead of comparing the blockhash against the emitted_blocks map
to see whether the block is part of the emitter's best chain, we
reduce the `last_mempool_tip` height to the last agreement height
during the polling logic.
The benefits of this is we have tighter bounds for avoiding re-
emission. Also, it will be easier to replace `emitted_blocks` to
a `CheckPoint` (since we no longer rely on map lookup).
