The wallet_datastore_first() SELECT statement only iterates from the
given key (if any), relying on the caller to notice when the key no
longer applies. (e.g. startkey = ["foo", "bar"] will return key
["foo", "bar"] then ["foo", "bar", "child" ], then ["foo", "baz"]).
The only caller (listdatastore) would notice the keychange and stop
looping, but reallly wallet_datastore_next() should do this. When I
tried to use it for migrations, I got very confused!
Also, several places want a simple "wallet_datastore_get()" function,
so provide that.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
e39c80bf8b