After some discussion with @shesek, and my own usage, we agreed that
a more comprehensive interface, which explicitly supports grouping,
is desirable.
Thus keys are now arrays, with the semantic that a key is either a
parent or has a value, never both.
For convenience in the JSON schema, we always return them as arrays,
though we accept simple strings as arguments.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
We add a generation counter, and allow update or del conditional
on a given generation.
Formalizes error codes, too, since we have more now.
Suggested-by: @shesek
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
