Adding a sync() function to filetarget (#293)

* Adding a sync() function to filetarget which runs on a timer to catch any events missed by fsnotify

* moving where we mark the final position to avoid a race condition exposed in testing, we want to stop the tailer before shutting down our tailer loop

* Fix changes from PR and refactor to remove circular imports and make config consistent

* Adding an "integration" type unit test which tests promtail as a whole

* fixing a bunch of imports I munged
moved tailer to its own file
refactoring filetarget to make synclogic easier to follow and reused as much as possible