Previously accounting tree and reference to accounts were pointer based.
This prevents us from freeing accounts at the end of each statement as
later statements of that transaction can potentially access memory and
the corresponding accounts. This resulted in a perceived memory leak as
the accounts were not freed per-statement.

In this implementation we convert pointers to index that can refer to an
array of accounts. This allows us to partition indexes based on
"generation" to define which accounts are live and which are already
dead. This is also a soft-pointer so we don't crash accessing it. This
also allows us to optimize the previous heavyweight "rollover" at the
time of generation overflow as we no longer need to detect the validity
of accounts based on generation, and rather we can just refer to logical
range of indexes to identify the liveness.

[#120028219]