Re-use/keep alive single storage plugin web resource

[exarkun]

The storage plugin web feature currently asks the storage plugin for its web resource every single time. This generally means there is one web resource per web request. This means the web resource can't have any persistent state unless it finds somewhere else to keep it.

It would be more convenient if the web resource could have persistent state on itself which was guaranteed to live as long as the node itself lives (which is approximately as long as the Python process lives).

An alternative to this would be for the plugin itself to keep the web resource alive and return it for repeated calls to get_client_resource. This is problematic because the node's whole configuration is a parameter to this method. The plugin would need to implement caching which correctly respects the node configuration which is non-trivial.

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/667

[exarkun]

Fixed in ​https://github.com/tahoe-lafs/tahoe-lafs/commit/0e82782b6166216c9e419d00cf6d0c325d86f6be