Update error messages.
Follow the coding style in C function naming.

Added lots of checks and nice error messages.
Added lots of tests

Disallow upsert in presence of a unique secondary key.

Update tests.
Fix typos.

_Noreturn kw must appear before the function name as per C11 grammar and
clang7 is the first one to enforce that.

Ecb_noreturn macro is defined using _Noreturn keyword if C11 support is
detected; otherwise __noreturn__ gcc attribute is used. The later must
appear after the function parameters list.  These two methods to
annotate a function are inherently incompatible in terms of the token
location relative to the function declaration.

Use gcc attribute exclusively since it is always available.

Conflicts:
	src/box/relay.cc

No semantical code changes.
struct replica will be used for applier + relay structure.

Conflicts:
	src/box/replica.h

Save received server_id and vclock to struct replica.

Remove unneeded C++ usage. This patch is needed to combine struct replica
and struct relay.

We already install signal handlers, but don't give them a chance
to execute, since don't yield during recovery. Add yields once
per a few tens of thousands of events.

Deploy a special hack with an idle watcher to make sure that
fiber_sleep(0) doesn't sleep for too long (1 ms without this
watcher).

libev sleeps for at least 1 ms (backend_mintime) unless there are
idle watchers. The logic of this behaviour is obscure to say the
least.

Since the upstream is also using *screw-the-community* cvs for
revision control, maintaining an own patch set on top of the
upstream is also a pain in the neck.

So, instead of patching libev (I hesitate to simply remove the
obscure logic since I don't understand the reason for it from the
comments, and I don't think making this logic optional is worth
the hassle, given also that we will diverge even more from the
upstream), create an idle watcher in fiber_sleep(), as a
workaround, to make sure backend_mintime is not looked at when
calculating sleep time for poll(), and zero timeout is used
instead.