Trans: New semantics of exceptions

Trans: New semantics of exceptions

Changed exception propagation rules according to the new semantics
documented in ucw/doc/trans.txt, which is based on transaction folding.

We avoid the (partially implemented) exception pool altogether, all
exceptions are just allocated from the pool of the current transaction,
and clever re-throwing rules make the pools last as long as they should.

Also, the current exception is kept track of in the current transaction
instead of the thread context.