Check TransactionServiceImpl::get_txcontext()

The method TransactionServiceImpl::get_txcontext() needs checking.
I'm not sure what semantics it should have when used for an "invalid",
e.g. committed, rolledback or marked rollback transaction?

I'm currently changing the method TransactionImpl.tx.getTimeLeftBeforeTimeout() to have clearer
semantics for use elsewhere, i.e.
TransactionImpl.tx.getTimeLeftBeforeTimeout(boolean errorRollback) throws RollbackException
with an error being raised if the transaction is not active and the parameter is true.

so this code becomes (for backwards compatibility)
try

{ pc.timeout = divideAndRoundUp(tx.getTimeLeftBeforeTimeout(false), 1000); }

catch (RollbackException e)

{ throw new UnreachableStatementException(); }

But as this stands, it could produce a negative or meaningless value in some circumstances?