A few changes and fixes to XML comments
TinyPerf: try to pass Slice by ref internally, whenever possible

- probably not worth it, but why not?
Added the IsolationLevel property on transactions which currently only return Serializable or Snapshot

- FdbIsolationLevel enum has same values as System.Data.IsolationLevel, but does not require a reference to System.Data.dll
- This could be used in the future by transaction wrappers or filters, to emulate other isolation levels (like read committed, ...)