MEDIUM: logs: buffer targets now rely on new sink_write

MEDIUM: logs: buffer targets now rely on new sink_write

Before this path, they rely directly on ring_write bypassing
a part of the sink API.

Now the maxlen parameter of the log will apply only on the text
message part (and not the header, for this you woud prefer
to use the maxlen parameter on the sink/ring).

sink_write prototype was also reviewed to return the number of Bytes
written to be compliant with the other write functions.