No more transaction_with or no Client::bind

[njaard]

I need to create a Portal, but I also need to set transaction properties ("start transaction read only deferrable"). The only way to create a transaction like that is with client.execute("start transaction ..."), but then I don't get a Transaction object, which means I can't do anything with a Portal.

[sfackler]

Ah yeah, we need to add an interface for custom transaction configuration.