getblockchaininfo: make bip9_softforks an object, not an array.

We can't change "softforks", but it seems far more logical to use tags
in an object rather than using an "id" field in an array.

For example, to get the csv status before, you need to iterate the
array to find the entry with 'id' field equal to "csv":

jq '.bip9_softforks | map(select(.id == "csv"))[] | .status'

Now:

jq '.bip9_softforks.csv.status'

There is no issue with fork names being incompatible with JSON tags,
since we're selecting them ourselves.

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