opt_mitosis: practically re-activate mitosis
opt_mitosis: practically re-activate mitosis

Since changeset b3e0b80a379a, mitosis was de-facto de-activated,
as the given formula resulted in more than one piece
only with rather huge amounts of data;
e.g., on a 16 GB machine, more than one piece was only used
with more than ~360M records, i.e., with SSBM lineorder table
of TPCH lineitem table of scale factor (SF) ~60 or larger.

Now, we activate mitosis in the followin three cases:

1)
The data size (assuming an operator with two arguments and one result,
each with a (0-byte) void head and an 8-byte lng tail) exceeds memory.
Then, when running with T threads we created P pieces
such that T pieces at a time fit in memory.

2)
Otherwise, if the table has more than MINPARTCNT (== 100000) records,
create pieces of size >= MINPARTCNT (to limit overhead), but at most
as mani pieces as we have threads.

3)
If merver5 is started with --forcemito, we aim at exploiting
full parallelism, i.e., as many pieces as threads, but avoid empty pieces.

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