Atomicity issues on node.js

[swansontec]

As reported at EdgeApp/edge-core-js#258, concurrent writes through the Disklet node.js backend can leave corrupted data on disk, like {"encryptionType":0,"iv_hex":"KmI7"}nbccfS5KIQIGo9k2iPW3Zp8pxV2Zg4XGo5sG3qHCU="}:

One approach is to serialize the node.js backend, so new work goes in a queue and we only execute one task at once. Another approach would be to write files atomically, maybe using a library to do it.

[swansontec]

Published the fix in v0.4.4.