HRMP channel deposits (#2225)

* Drive by fixes

The visibility modifiers are remnants of the previous structure where
HRMP wasn't a standalone module, by rather a submodule of the router
module.

* Add Currency assoc type to Config

This would allow us to reserve balance for deposits. This commit also
integrates the HRMP module in rococo, test-runtime and mocks to use the
balances pallet.

* Fix a bug that doesn't increment the age

In case the request is not confirmed, the age would be incremented but
not persisted.

* Fix cleaning the indexes

Before that change, the cleaning of the channel indexes was wrong, because it
naively removed entire rows that was pertaining to the para we delete.
This approach is flawed because it doesn't account for the rows that are
pertaining to other paras that contain the outgoing one.

This clearly violates the invariant imposed on the indexes, that all
the index rows must contain alive paras, but apart from that it also
lead to the situation where ingress index would contain the a different
set of channels that an egress have.

* 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.

* Tests: Adapt `run_to_block` so that it submits a proper config

* Tests: exercise the deposit logic