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.

In its current implementation, the heuristic tackles the use-case of
large arrays with many small append-only changes. In this case 'h' is
smaller in space than ZBlk0, but faster to read than ZBlk1. It does so,
by initally using ZBlk1 until a blk is filled up. Once a blk is full,
it switches to ZBlk1, as it was recommended by @kirr in
nexedi/wendelin.core!20 (comment 196084).

With this patch comes a test (bigfile/tests/test-zblk-fmt) that creates
benchmarks for different combinations and zblk formats. The test aims
to check how the 'heuristic' format performs in contrast to 'ZBlk0'
and 'ZBlk1':

---

Run append tests
---------------------------------------------
---------------------------------------------
Set change_percentage_set to 0.15
Set change_count to 500
Set arrsize to 500000
Set change_type to append

Run tests with format h:

	ZODB storage size: 318.565101 MB
	Access time: 0.747 ms / blk  (initially cold; might get warmer during benchmark)

Run tests with format ZBlk0:

	ZODB storage size: 704.347196 MB
	Access time: 0.737 ms / blk  (initially cold; might get warmer during benchmark)

Run tests with format ZBlk1:

	ZODB storage size: 163.367072 MB
	Access time: 74.628 ms / blk  (initially cold; might get warmer during benchmark)