enhancing sincedb flushing

[bdanziger]

enhancing sincedb to properly flush when files have been read from, while preserving the sincedb_write_interval

[bdanziger]

Today, the watch.rb only returns to the caller after file creation, or modification. So, if the caller needs to do something, you are out of luck. Example : tail.rb, uses sincedb_write_interval and only writes once during the interval, even though there might have been read events after the sincedb flush but before completion of the interval. So, returning back to tail.rb would allow the tail to flush when the interval expires. Additionally, I moved the @sincedb_last_write to the routine that actually does the write. It is the only routine that really knows when it is written and should control this variable.

[guyboertje]

I intend to bear this in mind when I refactor the whole sincedb behaviour in response to #75