Integrate initial payment information into a subscription state by extending subscription period

[exarkun]

With #748 done, we will have a stream of payment information flowing. Create a service into which it can flow.

This service is responsible for a couple things, one of which is the maintenance of an existing subscription when a matching payment is detected.

It must be possible to associate this payment with an existing subscription. With Sapling diversified payment addresses, an option is for each subscription to receive its own payment address. Prior, an option is for the secure message field of the transaction to include a subscription identifier (returned previously alongside the other Tahoe-LAFS configuration data).

See also #750

[exarkun]

Migrated to S4-2.0 repository.