Local implementation is not atomic, isolated, durable

[urisimchoni]

The general expectation from a blob store is that PUT operations for single files are atomic, isolated, and durable:

• either the whole PUT succeeds or the file keeps its old content
• none sees the effect of the PUT until it's complete
• a successful put means the data is stored in some non-volatile way

The current Local adapter doesn't fulfill any of those because it truncates the file at the beginning of the PUT if it already exists, and because it doesn't call Sync.