forthcoming refactoring actions, following review of #368

[fredbi]

Reviewers comments from #368, left as TODOs, in a nutshell:

• Confusing/pointless use of type alias for bool

My point was just to replace this by:
func MultiPut(ctx context.Context, stores []MultiStoreUnit, name string, buffer []byte, doesNotExist bool) error

• separate porcelain funcs from cmd

refactor types defining such dsl-ish things to a separate package
Yes. Indeed this is the goal I am pursuing. I'd like many little things like this (including the whole flagsT, with default settings and GCS strategy) to move to some pkg/api/helpers or pkg/sdk/helpers package to be available as an higher level API for instance, a python wrapper. Please suggest a name for that.

• func localPath(consumable fmt.Stringer) (string, error)

functions or methods defined in storage
remains a questionable design decision

• cli_fuse_test.go: func testContext() string
  cruft?
• I'd really like to move all this to some pkg/fuse package.

creating a package for the filesystem implementation stuff has been a todo for a while, although idk if we have an explicit issue for it.

• Maybe the log level is best placed as a persistent flag under root?
• CLI fuse_test: assert stdout & stderr separately, not through a combined output
• clarify impact & priority of env var GOOGLE_APPLICATION_CREDENTIALS (re remark from Ransom)