Rationale:
Any error which implies a system administrator action needs to be
marked explicitly in the log, so that it's easy to find and act
upon.

This change the number of log levels, so the default log level was
bumped from 4 to 5. Installations which set the log level
explicitly need to adjust their setting.

Implement blueprint
https://blueprints.launchpad.net/tarantool/+spec/wal-mode-none

Add wal_mode = none setting.
It's possible to chagne wal_mode without restart.

When wal_mode = none, we don't write to WAL.
It's still possible to take a snapshot.

Replication and hot standby are currently not
operational in this mode.

Do not use bundled libobjc on Darwin.
Refactor cmake scritps: introduce os.cmake, arch.cmake,
compiler.cmake, libobjc.cmake, luajit.cmake.

Remove the hack with building libobjc/luajit as a
dependency on cfg.

Do not specify -ggdb when compiling with clang.

Add -fno-objc-legacy-dispatch and -fobjc-nonfragile-abi
when compiling with CLang on Linux.

Remove -fgnu-runtime when compiling with gcc on Mac.

Many minor cleanups.

Add a makefile to libobjc.

script_dir option defines path to a directory which contains the init.lua file.

Make snap_io_rate_limit and io_collect_interval dynamic.
Add a test case.
Update the user guide.

Update the manual and the configuration file
to reflect the new WAL writer architecture.

This reverts commit 8dc70aac.

This patch introduces wal_mode configuration variable,
which can take either "fsync" or "fsync_delay" values.

In "fsync" mode, we open the write ahead log in O_SYNC
mode, and thus sync every write to disk.

Th fsync_delay mode is the backward-compatible mode:
in this mode, we only do fsync once every fsync_delay
seconds. If fsync_delay is 0, there is no fsync.

Otherwise, this patch is only adding auxiliary headers,
object files, etc.

paramaters use floating point representation.

lua.result was somehow missed in the last commit, add now

https://blueprints.launchpad.net/tarantool/+spec/namespace-to-space

Rename 'namespace' to 'space' everywhere:
in configuration, in implementation, in documentation.

Now that confetti supports it, use boolean for true/false
yes/no config options.
Update the manual accordingly.

Remove auto-generation of tarantool_box_cfg.cfg.
This was intended as a sample configuration
file, but it is no longer syntactically correct,
and thus can not be used.
Besides, there are now semantical rules on top
of the syntax restrictions (e.g. you can't define
memcached_expire on a replica), which also
make the auto-generated config incorrect.

Remove it to not mislead first-time users into
trying it out.

Test enviroment:
  - start/stop server routine was updated.
Tests:
  - fix double start srvers.

* valgrind suppresions update

* core signal handling sigaction -> libev
* suppresion file updated

* test-suite valgrind support