Reserve currency for opening the channels

Note the ugly `unique_saturated_into` calls. The reason for them is the
currency trait accepts and defines the `Balance` associated type and the
deposit values are coming from the `HostConfiguration` where they are
defined using the `Balance`.

I figured that parameterising `HostConfiguration` would be annoying. On
the other hand, I don't expect these `unique_saturated_into` calls to
give us problems since it seems to be a reasonable assumption that this
module will be instantiated within a runtime where the Currency provided
will have a Balance that matches the one used in the configuration.