Buffered input plugins

Input plugins may emit events one-by-one (for example, `in_exec`), but it degrades total performance of Fluentd event routing.
If we can support buffer plugins in input plugins, it can improve performance with users' configurations for such cases.

It may be implemented as:
- an extended API (method) for existing input plugin class (if plugin implements it, input plugin can work with buffer)
- a plugin helper (or a new method of event emitter plugin helper)

Output plugin supports both of buffered and non-buffered way. So Input plugin SHOULD support both too.