0.2.0: Asynchronous sendStatus(), pass WAL position to handler, fix plugin a…

…rgs etc (#2)

* Remove vendor folder

* don't drop the replication slot

* Fix pluginArgs

* relation: change to pointer, add Get(), fix sendStatus()

* pass wal position to handler

* send status messages async, fix example, other stuff

* Fix test

* revert timeouts

* Add Flush() function. By default send status msgs only with write walPos

* Add walRetain option

* Take manual flushes into account

* check for nil message

* make relation non-pointer, fix misc

* fix relation Get

* add relation.IsEmpty()

* change relation.Get

* Cosmetic changes

* Add connInfo to NewRelationSet(), fail on handler knob, cleanups for PR