There are two formats to save data with a ZBigFile: ZBlk0 and ZBlk1.
They differ by adjusting the ratio between access-time and growing
disk-space, where ZBlk1 is better regarding to disk space, while ZBlk0
has a better access-time. Wendelin.core users may not always know yet or
care which format fits better for their data. In this case it may be
easier for users to just let the program automatically select the ZBlk
format. With this patch and the new 'h' (for heuristic) option of the
'ZBlk' argument of ZBigFile, this is now possible. The 'h' option isn't
really a new ZBlk format in itself, but it just tries to automatically
select the best ZBlk format option according to the characteristics
of the changes that the user applies to the ZBigFile.

With this patch comes a test (bigfile/tests/test-zblk-fmt) that creates
benchmarks for different combinations and zblk formats. The test aims
at checking that the 'heuristic' format behaves mostly as good as the
explicitly set formats:

Use only a very small change size, so that heuristic always uses ZBlk1
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Set change_size_set to 20
Set change_count to 500
Set arrsize to 1000000

Run tests with format h:

	ZODB storage size: 20.388751 MB
	Access time: 3.83012294769

Run tests with format ZBlk0:

	ZODB storage size: 1064.636095 MB
	Access time: 3.63488578796

Run tests with format ZBlk1:

	ZODB storage size: 18.59421 MB
	Access time: 3.93918204308

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Use only a very big change size, so that heuristic always uses ZBlk0
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Set change_size_set to 200000
Set change_count to 500
Set arrsize to 1000000

Run tests with format h:

	ZODB storage size: 2113.32534 MB
	Access time: 3.79592084885

Run tests with format ZBlk0:

	ZODB storage size: 2113.254473 MB
	Access time: 3.76431703568

Run tests with format ZBlk1:

	ZODB storage size: 1651.236315 MB
	Access time: 4.11528992653

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Mix between change size so that heuristic switches between ZBlk0 and ZBlk1
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Set change_size_set to 20,200000
Set change_count to 500
Set arrsize to 1000000

Run tests with format h:

	ZODB storage size: 820.17736 MB
	Access time: 3.85217094421

Run tests with format ZBlk0:

	ZODB storage size: 1576.361791 MB
	Access time: 3.65322995186

Run tests with format ZBlk1:

	ZODB storage size: 815.323463 MB
	Access time: 3.96401691437

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