Fixing debug journal:

* mark when a transaction is being disposed to avoid race conditions in debug journal's records
* mark flushing operations if they are called by async background task
Better field name
Making work with the debug journal a bit easier
Adding two passing tests that exposed RavenDB-2806 (already fixed). Changes we made then in order to repair that also fixes these two tests.
RavenDB-2891 Failing tests that exposes the issue of writing overflow pages to the data. The following conditions need to be fulfilled in order to replicate that:

1) there are at leas two journals
2) journals' page translation tables contain records as follow:
* 1st PPT:
	#3 - free marker
	#4 - non free
* 2nd PTT:
	#3 - non free overflow (size 2)
	#4 - free marker

Then during applying pages from such journals (by background operation or during an incremental backup restore) the page #3 will be overwritten by page #4 from 1st journal.
RavenDB-2891 Filtering out pages that has been freed and writing them to the data file caused overwriting of overflow pages.
Merge pull request #1534 from arekais/RavenDB-2891

RavenDB-2891