RavenDB-2268 More fixes to get prefixed keys working (20 failing tests at this point)
RavenDB-2268 Do not increase the AvailablePrefixId value in the page header immediately because in case of a page splitting it can happen that there is no space and additional splits are needed (previous implementation broke the parent page when AddSeparatorToParentPage was called)
RavenDB-2268 Clear prefix offsets in a temp page header
RavenDB-2268 Fixing comparison sign
RavenDB-2268 Clear prefix info for a new allocated page