ryu.log: change type of --use_syslog option to Boolean.

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$ ryu-manager --help
(....)
  --use-syslog USE_SYSLOG
                        output to syslog

$ ryu-manager --use-syslog
usage: ryu-manager [-h] [--version] [--verbose] [--noverbose] [--use-stderr]
(....)
ryu-manager: error: argument --use-syslog: expected one argument
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Signed-off-by: EisukeTAKAHASHI <takahashi.eisuke@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Add request/reply event support to support communication between apps

What most of applications need to do is:

1) register a handler to catch a specific event
2) The handler does multiple asynchronous tasks (query dbs, configures
switches, etc).

2) can be implemented in an asynchronous way, that is sending multiple
requests at the same time. However, I don't think most of developers
need to do or want to do. The API to handle multiple asynchrnous tasks
in a synchronous way is more handy.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>