ISPN-2840, ISPN-3235 and ISPN-3236

Improves and fixes for Repeatable Read isolation level (with and without write skew check)

https://issues.jboss.org/browse/ISPN-2840
https://issues.jboss.org/browse/ISPN-3235
https://issues.jboss.org/browse/ISPN-3236

Transaction experts are welcome to look at it :)