Mainly fixing type annotations, but some real fixes:
1. GossmapHalfchannel.from_str() should be a classmethod.
2. update_channel had weird, unusable default values (fields can't be NULL,
since we use it below).
[ There was one more occurence where isinstance should be used above
type() == xyz comparison. -- MS ]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>