Ensure IStorageServer-using code doesn't have interactions that conflict with HTTP implementation

[itamarst]

On the assumption that we keep IStorageServer in place, and provide a HTTP implementation of the interface, current users of the interface need some updates.

In particular, some parts of the code make unwarranted assumptions about interaction patterns, e.g. that if command A is run before command B then command A will be started before command B. With an HTTP client, these commands will be sent in parallel, not necessarily over the same TCP connection as Foolscap does, so this assumption won't always be true.

For this particular example, the code would need to be modified so that B was only sent when a response is received from the A command, ensuring the required ordering.

[itamarst]

To go the manual audit route, given a whole bunch of arbitrary fakes:

1. For each method in IStorageServer and related objects (bucket reader/writer), grep for method definitions; eventually this should discover all fakes. E.g. allmydata.test.mutable.util.FakeStorageServer? is not annotated with the IStorageServer interface but implements it.
2. For each fake, temporarily add logic to print tracebacks(?) to file.
3. Temporarily add logic to print tracebacks to file for real code too.
4. Run unit and integration tests.
5. Uniquify file.
6. Permanently add interface implementation annotation, so in the future we can just use grep.

At this point we should have locations of all calls into Foolscap storage protocol, and can audit it for bad assumptions like "order of calls matches order of delivery".

[itamarst]

Could be expanded to removing use of callRemote/callRemoteOnly on the read/write buckets (and replace all use of callRemoteOnly while we're at with callRemote+log-on-error). or that could be a separate ticket.

[itamarst]

Probably easiest to just fake callRemote in the IStorageServer adapter, but I did get rid of all callRemoteOnly.

[GitHub <noreply@…>]

In 2eafe41/trunk:

Merge pull request #1117 from tahoe-lafs/3779-istorageserver-with-fewer-assumptions

Reduce implementation-dependency of IStorageServer

Fixes ticket:3779