Making sure that there can only be a single flushing operation at one time
Merge branch 'voron' of github.com:ayende/raven.voron into voron
Merge branch 'voron' of github.com:ayende/raven.voron into voron
Merge branch 'voron' of github.com:ayende/raven.voron into voron

Conflicts:
	Voron/Impl/Journal/WriteAheadJournal.cs
	Voron/StorageEnvironment.cs
Fixing the merge, we already already doing locking in journal applicator now.
Minor
We _have_ to lock on disposal of WriteAheadJournal