BufferOut
is a storage where the objects stay before they have been sent.
A buffer out can hold persistent or non-persistent buffer objects, made
by a BufferObjectFactory.
When it is persistent, a newly created buffer out first restores its
transaction id and then restores its persistent objects (only if their
idTransaction is lower than the buffer's transaction id).
The BufferOutManager is node-specific and keeps track of all existing
buffers.