Squashed 'RavenDB/Server/Raven.Voron/' changes from 93a2e24..92f75a1

92f75a1 support writing to multiple trees in single batch

git-subtree-dir: RavenDB/Server/Raven.Voron
git-subtree-split: 92f75a13b6b12fa7ce7ea844dd4fc6afca5063ba
Merge commit '6cc5c76250371a1c4401224effeb0c5922c05afd' into 3.0-voron