Define a server-side interface for plugins for storage economics

[exarkun]

The main premise here is that being able to hook into the RIStorageServer interface and choose to allow or deny operations on a case-by-case basis along with a minimal amount of additional information will allow storage server operators to enforce access control more conveniently.

The goal is that operators will be able to codify certain properties of the economic model to which they are subject in a plugin which uses these hooks to allow only access which is economically feasible for them.

In principle there is likely no reason the plugin has to implement an *economic* policy. For example, it could enforce a web of trust policy or some kind of friend-net. However, the focus of initial development on the interface will be economic. The hope is that by facilitating economic interactions, there will be more incentives to become a Tahoe-LAFS storage operator.

[exarkun]

Efforts in ​https://github.com/tahoe-lafs/tahoe-lafs/compare/3049.storage-economics-plugin

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/619

[exarkun]

Fixed in ​https://github.com/tahoe-lafs/tahoe-lafs/commit/c2217fece7dca0d4a2c961e22da3aa689fa43e5e