RavenDB-2268 Limiting the number of PrefixedSlice allocations
RavenDB-2268 Getting rid of the needless Slice value in the PrefixNode
RavenDB-2268 Limit number of allocated PrefixNode instances
RavenDB-2268 Small fixes
RavenDB-2268 Make sure we don't override PrefixedSlice.Empty
RavenDB-2268 Less PrefixedSlice allocations caused by page splits