Do not store context in structs, and other fixes

[gammazero]

Stop storing context in structs, and instead pass a context as a function argument in the places where support for cancellation is needed.

Much of the functionality does not need to support cancelation, and even in some cases where a request may have been canceled, letting the function complete is preferable to avoid partial updates to MFS. Cancellation only remains in places where a caller may be waiting for a signal that something has completed.

• Handle additional TODO items in code
• Fix potential race condition
• Close() always stops republisher; fixes possible goroutine leak
• Clear dir entries cache after sync().
• Republisher can pick up new values during the retry loop.

This PR will replace #58

Part of the motivation for this PR is in preparation for moving this package into a new repo shared by go-ipfs-files and go-unixfs, so that fixes can be reviewed before placing files in a new repo. This PR can be canceled if a more complete rewrite is preferred to fixing the issues handled by this PR.

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[gammazero]

I am concerned about is that this PR makes breaking API changes. If that is going to be a problem, then I can make a smaller PR with just the fixes and TODO items, and save the API changes for when this moves to another repo.

[gammazero]

Closing to save API changes for a later incarnation of mfs.