Conflicts:
	src/box/sophia_engine.h

* transaction is yet active on CALL return
* transaction conflicts on commit

Conflicts:
	src/box/replication.cc

closes gh-771

Conflicts:
	doc/user/configuration-reference.xml
	doc/user/connectors.xml

Conflicts:
	doc/sphinx/book/connectors/index.rst

Mostly remove obsolete statements. Document slab_alloc_maximal.

~/.config/tarantool superseeds /etc/default, not augments it

Conflicts:
	src/CMakeLists.txt
	src/lua/init.cc
	src/lua/log.lua

Don't run event loop if socket is ready to write. *Always* write
optimistically. With fiber.channel(), channel push  wakes up the waiter
without in the same event loop iteration. This means that whenever we have
data on an incoming connection, we can push messages to outgoing connections
in the same iteration, without epoll_ctl syscalls or unnecessary
epoll_wait.
With 1 incoming and 5 outgoing connections, for example, this saves
~10 syscalls per each incoming request.

If a fiber becomes ready on a user-defined event, schedule it
in the same event loop, not in the next loop, to save one epoll_wait()
invocation in cases when something is added to a fiber channel,
and there is a fiber ready for execution waiting on the channel.

Add a micro-bench to the unit test suite.

Rollback d29469f2, it makes build bug on FreeBSD build and doesn't affect on OSX