Add flush delay

[Arkoniak]

Well, idea is quite simple:

flush option instead of true/false should have three values:

• instant
• auto
• no

instant and no should work in the same way, as true/false is currently working. auto on the other hand should save the time of last flush and if the time of the logging event exceeds some threshold (for example 1 second) then a new flush is made and last flush time is renewed.

It may have a drawback that if you have a burst of logging events and then a long pause, few of the last events wouldn't show up until new log event or when stream is closed, but I think it's fine.

Threshold time should be set with the help of flush_auto_threshold argument.