allow uncoordinated reads and writes of a mutable file or directory locally

[davidsarah]

In the fix to #265 in Tahoe v1.1, MutableFileNodes were made to serialize writes to a given mutable file or directory by a single gateway / storage client.

As mentioned in ticket:265#comment:4, this does not apply to concurrent reads and writes -- the read may fail rather than obtaining some snapshot of the file or directory.

Clients using filesystem-like interfaces such as SFTP or FUSE may not expect such failures. At the time of writing, the SFTP documentation at wiki:SftpFrontend incorrectly claims that readers will get a snapshot of the file. (For mutable files, this problem applies directly to the file; for immutable files it applies to the parent directory when that is mutable.)