Transactions: retrieve transaction from upstream pipelet transactions; add xs_tests core ToDos
Pipelet..emit_operations(): use transactions instead of more()
Transactions: get_options() sets options.more using count, removed.length, and added.length; throws exception if closed; get_emit_options() calls get_options() to test closed and get "more"; Pipelet..more removed; Pipelet.emit() optimized; Pipelet.emit_operations() streamlined
Pipelet.._subscribe(): use transactions