net: buf: Redesigned API with split data and meta-data

Redesign of the net_buf_simple and net_buf structs, where the data
payload portion is split to a separately allocated chunk of memory. In
practice this means that buf->__buf becomes a pointer from having just
been a marker (empty array) for where the payload begins right after
the meta-data.

Fixes #3283

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>