Don't log error on missing blob length

Since 6.0 HF33 Nuxeo does not rely on a file length provided by the binary manager. This causes side effects for certain scenarios used by customers for ingestion:

• ERROR is logged each time the blob is read
• the length has to be computed by reading the blob which is inefficient most of the time

Provide a fix that

• allows use of blobs without triggering "Missing blob length" errors (perhaps lower this exception's priority to Warning)
• allow writing new blobs from InputStreamSource without performance degradation