[kernel,server] add thread pool for main server.
[kernel/net] note about multi-thread server - tests/benchmarks (see 0a01f79559)
[db] drop extra inclide from Avltree
[db] add thread-safity for db wrappers, using boost::shared_mutex.

bc-benchmark shows not bad results, with 3 workers/threads:
HSET: 2.6 times faster
HGET: 2.7 times faster

See also 0a01f795595.
[kernel] add const specifier for Commands methods.

- find()
- notImplementedYetCallback()
[utils] one_conn_awk_bench: add progress bar

Useful to track when server stop reponding because or segfaulted main
server of multi-thread server, that was introduced first in 0a01f795595.

bc-benchmark not cause segfault server, but current one does
(one_conn_awk_bench)
Seems that because of one connection to server (in current script), and
it try to reuse one session for many threads, which is not good.
[kernel] commandserver: change TODO about test/bechs, I've already tested it.

See also:
 - 8d27bb52a8 - bench
 - 8599f0b708 - note added
[kernel] commoncommands: add note that multi-thread server is not final.

A lot of work need to do.
[kernel] commandserver: rename m_socket into m_ioService
[kernel] reimplement multi-thread server using pool of io services.

Previous implementation has a bug, with one connection to server, it try
to reuse one session for many threads, which is not good. (see 27faeab692)

So I decide to use pool of io_service based on
boost_asio/example/http/server2/io_service_pool.hpp
And it fix the initial issue.

First multi-thread server was introduced in 0a01f7955.

Speed of this implemention like in previous (multi-thread one) see
8d27bb52a8c for more info.
[kernel] ioservicepool: rewrite using std::thread instead of boost::thread
[test] run multi-thread server (from 8134220634)