Add read-only resource manager to ChannelManager

Integrates the ReadOnlyResourceManager into the ChannelManager. This can
be enabled by the enable_resource_manager field added in the UserConfig.

The main 4 call sites are when adding/removing channels and add/resolve
forwarded HTLCs. The add_htlc call into the resource manager is done before
the queue_add_htlc on the outgoing channel.

In read-only mode, it will only log the forwarding outcome suggestion
from the resource manager and use the accountable signal suggested.

Note that it only works in std given that we need access to the time.

Author: elnosh