Removed wrap_with argument from new and new_or_existing methods
of Transient Data Containers.

Removed delete method of Transient Data Containers.

Added out-of-memory protection to Transient Data Containers.  A
  new __init__ value ('limit') is used to specify the max number
  of objects that can be contained within a transient data container.
  A new envvar ZSESSION_OBJECT_LIMIT can be used to control the
  limit of the default session_data TDC.  Also updated help and
  API docs with this change.

Added a new exception, MaxTransientObjectsExceeded, which is raised
  when the OOM protection kicks in.

Various implementation changes including the use of a BTrees Length
  object to store Transient Data Container length info as well
  as improvements to how buckets are expired.

Addition of tests for OOM protection fatures.