buffered producer subclass
make buffering a counter instead of a boolean
buffered_do, works like DBIC's txn_do
prevent calling send_buffered inside buffered_do

it does not make much sense, and it makes the "rollback" much too
complicated
documentation

also, make most of ::Buffered private
rename ::Buffered -> ::Transactional (and methods)
better transactional semantics

fully re-entrant, stack-based
changelog
changelog bump