jsonrpc: make command structures const

Needs a slight tweak to test/mockup.sh to find them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
jsonrpc: add getblock command

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
protocol: rename depth to height

Doesn't change the protocol on the wire, just the field names.

In bitcoin, "depth" usually means number of confirmations; "height" is
used to mean number of blocks since the genesis block.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
jsonrpc: add getblockhash command

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>