Support the following configuration:

    box.cfg {
        replication_source = {
            "uri1";
            "uri2";
            "uri3";
            -- ...
        }
    }

This change is backward-compatible: old syntax is supported as well.
box.info.replication is not changed and displays state of the first
(if sort by URI) master.

This patch also adds a simple hack to automatically detect the first
master (if sorted by URI) during initial cluster bootstrap by
comparing box.cfg.listen uri with replication_source uris.

Remove broadcast(): it din't work as expected and wasn't used
by anyone.

Enable monotonic timers (if available).

Enable floor() usage in computations (instead of a crude approximation
with bithacks).

Enable eventfd and signalfd usage on Linux.

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.