…after write.

This reworks the createKeyFileWithLock function (used only by
writeStreamWithLock) so it always writes to a temporary file, which
is renamed by writeStreamWithLock when complete. A deferred recover is
added to clean up the temporary file if writing fails. This essentially
deprecates the TempDir option by making all writes atomic.

… BasePath.

The key walking functions have been modified to ignore everything inside that directory, so getting a list of keys while a WriteStream is in progress will not include the temporary files.