Add --backlog option to change listen backlog from command line.

The listen backlog is currently fixed to the SOMAXCONN value, which is 128.
SOMAXCONN is sometimes not enough for heavy load servers, so this options increases
some capacity under some conditions.
start_server: Add --backlog option description to POD
Merge pull request #12 from yuryu/feature/option-backlog

Feature/option backlog