SPV clients need to poll one or more block sources for the best chain
tip and to retrieve related chain data. The Poll trait serves as an
adaptor interface for BlockSource. Implementations may define an
appropriate polling strategy.
Implements a simple HTTP client that can issue GET and POST requests.
Used to implement REST and RPC clients, respectively. Both clients
support either blocking or non-blocking I/O.
Defines an interface and related types for fetching block headers and
data from a block source (e.g., Bitcoin Core). Used to keep lightning in
sync with chain activity.