enable some asynchronous logging

[mxk1235]

AsyncRawSocketSender is just a wrapper around RawSocketSender that does its work in a separate thread.

also added a factory method to create a logger with its own sender, and moved from using a synchronized method to using a synchronizedMap for the loggers.

[komamitsu]

Thanks for the PR and sorry for my late reply.

I looked at the PR and I found that AsyncRawSocketSender.emit() and flush() create anonymous Runnable every time. It can cause a poor performance.

So I think we'd better use consumer-producer model to implement async sender. What do you think?

[mxk1235]

sorry for the late reply. yes, performance might be an issue. but i think i'd rather let someone else do that refactoring. i am currently in the middle of a different task at work, which no longer has anything to do with AsyncRawSocketSender.

[mxk1235]

changed to nested classes.

[anusricorp]

@komamitsu @mxk1235 can this PR be revived? Could we handle the concerns Mitsunori Komatsu had