Merge remote-tracking branch 'origin/1.6' into gh-806

.travis.yml

debian/tarantool-common.install

extra/dist/CMakeLists.txt

extra/rpm/tarantool.rpm.spec.in

src/CMakeLists.txt

src/backtrace.cc

src/backtrace.h

src/box/alter.cc

src/box/box.h

src/box/engine.cc

src/box/key_def.h

src/box/memtx_engine.cc

src/box/port.h

src/box/recovery.cc

src/box/recovery.h

src/box/request.cc

src/box/sophia_engine.cc

src/box/space.h

src/box/wal.cc

src/box/wal.h