Give ChannelManagerReadArgs HashMap-of-monitors ownership

Give ChannelManagerReadArgs HashMap-of-monitors ownership

Its somewhat awkward that ChannelManagerReadArgs requires a mutable
reference to a HashMap of ChannelMonitors, forcing the callsite to
define a scope for the HashMap which they almost certainly won't use
after deserializing the ChannelManager. Worse, to map the current
version to C bindings, we'd need to also create a HashMap binding,
which is overkill for just this one use.

Instead, we just give the ReadArgs struct ownership of the HashMap
and add a constructor which fills the HashMap for you.