doc: describe throttling
all: release light0.1beta0.15
trans_logger: fix potential race
net: also propagate foreign symlinks
all: replace ref_skip_sync with flags, refine completion semantics
infra: remove deprecated tracing
infra: remove deprecated brick_atomic.h
infra: remove obsolete brick_locks.h
infra: remove GFP_MARS
infra: move power led implementation to strategy layer

The interface must remain callable by each worker brick, but
the implementation belongs to the strategy layer (preparation for
MARS Full).
trans_logger: remove obsolte CONFIG_MARS_LOGROT

Always on. OFF makes no sense at all.
brick_mem: remove obsolete CONFIG_MEM_RETRY

This is always on. The code won't work without it.
all: remove obsolete NULL ptr checking
all: remove some dead code
all: remove deprecated MARS_IO() debugging
infra: move init/exit code from brick.h to brick.c

Avoid code duplication when multiple brick types are used later.
infra: mark accessibility of members
infra: hide some brick internals
if: remove deprecated timer
if: simplify unplugging