Avoid connection-per-RPC-call again by caching connections

Avoid connection-per-RPC-call again by caching connections

In general, only one request will be in flight at a time in
`lightning-block-sync`. Ideally we'd only have one connection, but
without using the `futures` mutex type.

Here we solve this narrowly for the one-request-at-a-time case by
caching the connection and takeing the connection out of the cache
while we work on it.