read/write data from clients in worker threads

Currently, we read/write data from clients in the main thread in
the non-blockng way.  This has the following problems:

 - The implementation is much more complex than doing it in the
   blocking way.
 - The main thread is a bottleneck when doing a large data I/Os
   (e.g. object recovery does a lot of 4 MB reads and writes).

This patch creates one worker thread for each connection, makes socket
descriptors blocking, and reads/writes data from clients in the worker
threads in blocking way.

With this patch, client_handler() doesn't print warnings of the
loop_check tracer any more.  I did the following benchmark to emulate
a VM I/O while object recovery, and could see a lot of performance
improvement.

[benchmark]

  Simulate object recovery with 10 concurrent 4 MB reads, and simulate
  VM I/Os with 4 KB reads.

  $ ./script/vditest -C 10 -B 4M -T 10 test > /dev/null & \
    ./script/vditest -B 4k -T 10 test

[result (before)]

  options: -B 4096:4096 -c writethrough -C 1 -D 100:0 -o 0
           -p linear -s 1376684853 -S 0:4294967296 -T 10 -f 0
  Total read throughput: 638156.8B/s (623.2K/s), IOPS 155.8/s.

[result (after)]
  options: -B 4096:4096 -c writethrough -C 1 -D 100:0 -o 0
           -p linear -s 1376684811 -S 0:4294967296 -T 10 -f 0
  Total read throughput: 4237312.0B/s (4.0M/s), IOPS 1034.5/s.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>