Smesher api implementation

[avive]

Please don't merge yet - WIP

[avive]

Trying to subscribe to the PostDataCreationProgress stream from a client results in this error:

$ pos-stream-progress
ERRO 002 13:53:42.368 repl.printPostDataCreationProgress ▶ error reading from post data creation stream: rpc error: code = FailedPrecondition desc = no post data creation session is in progress

Looks like node returns this wrongly. Clients should be able to subscribe to this stream at any time, including before starting to create pos data and node should not return an error on that stream - it should only return pos progress messages on it.

[moshababo]

Looks like node returns this wrongly. Clients should be able to subscribe to this stream at any time, including before starting to create pos data and node should not return an error on that stream - it should only return pos progress messages on it.

As discussed, this protection was removed in the latest implementation. The stream now hangs until session starts.

[moshababo]

c3d85d5 changes:

• /common/storage.go: os.WriteFile / os.ReadFile -> ioutil.WriteFile / ioutil.ReadFile.
  I’m not sure how it previously passed compilation.
• Support spacemeshos/api@81ae866 changes, specified here: New Smesher API api#145 (comment)
• In printPostStatus, remove the reliance on the "pos-data.json" file. The returned status now returns this data, so clients won't need to maintain it for this purpose.

[avive]

@moshababo - this PR is ready for review