Retrieving a full block uses RPC command

[Danconnolly]

Using the RPC command fails with large blocks. The hex encoded block appears to be limited to about 2GB (hex encoded = 4GB).

Retrieving from the rest interface using binary is much more efficient.

The first time the call is made to retrieve the block the code should check if the REST interface is enabled. If not then a warning should be emitted.

When a block is retrieved, it should prefer the REST interface if it is available.