Various fixes and improvements.

* AssertInTransaction added for more readable code.
* ShieldedSeq and ShieldedTree modified (and fixed) to match policy of the rest - reads work outside of transactions, but for modifications the caller must explicitly open one.
* BetShop test became much slower due to in-transaction reading from Seqs. However, this had to be changed. Must improve Enlist() and such.
Merge branch 'master' of https://github.com/jbakic/Shielded