With zwrk for ZODB being similar to what wrk is for HTTP.

Rationale: simulating multiple clients is:

1. noisy - the timings from run to run are changing sometimes up to 50%
2. with significant additional overhead - there are constant OS-level
   process switches in between client processes and this prevents to
   actually create the load.
3. the above load from "2" actually takes resources from the server in
   localhost case.

So let's switch to simulating many requests in lightweight way similarly
to how it is done in wrk - in one process and not so many threads (it
can be just 1) with many connections opened to server and epolly way to
load it with Go providing epoll-goroutine matching.

Example summarized zbench-local output:

	x/src/lab.nexedi.com/kirr/neo/go/neo/t$ benchstat -split node,cluster,dataset x.txt
	name                             time/object
	cluster:rio dataset:wczblk1-8
	fs1-zhash.py                             23.7µs ± 5%
	fs1-zhash.go                             5.68µs ± 8%
	fs1-zhash.go+prefetch128                 6.44µs ±16%
	zeo/py/fs1-zhash.py                       376µs ± 4%
	zeo/py/fs1-zhash.go                       130µs ± 3%
	zeo/py/fs1-zhash.go+prefetch128          72.3µs ± 4%
	neo/py(!log)/sqlite·P1-zhash.py           565µs ± 4%
	neo/py(!log)/sql·P1-zhash.py              491µs ± 8%
	cluster:rio dataset:prod1-1024
	fs1-zhash.py                             19.5µs ± 2%
	fs1-zhash.go                             3.92µs ±12%
	fs1-zhash.go+prefetch128                 4.42µs ± 6%
	zeo/py/fs1-zhash.py                       365µs ± 9%
	zeo/py/fs1-zhash.go                       120µs ± 1%
	zeo/py/fs1-zhash.go+prefetch128          68.4µs ± 3%
	neo/py(!log)/sqlite·P1-zhash.py           560µs ± 5%
	neo/py(!log)/sql·P1-zhash.py              482µs ± 8%

	name                             req/s
	cluster:rio dataset:wczblk1-8
	fs1-zwrk.go·1                              380k ± 2%
	fs1-zwrk.go·2                              666k ± 3%
	fs1-zwrk.go·3                              948k ± 1%
	fs1-zwrk.go·4                             1.24M ± 1%
	fs1-zwrk.go·8                             1.62M ± 0%
	fs1-zwrk.go·12                            1.70M ± 0%
	fs1-zwrk.go·16                            1.71M ± 0%
	zeo/py/fs1-zwrk.go·1                      8.29k ± 1%
	zeo/py/fs1-zwrk.go·2                      10.4k ± 2%
	zeo/py/fs1-zwrk.go·3                      11.2k ± 1%
	zeo/py/fs1-zwrk.go·4                      11.7k ± 1%
	zeo/py/fs1-zwrk.go·8                      12.1k ± 2%
	zeo/py/fs1-zwrk.go·12                     12.3k ± 1%
	zeo/py/fs1-zwrk.go·16                     12.3k ± 2%
	cluster:rio dataset:prod1-1024
	fs1-zwrk.go·1                              594k ± 7%
	fs1-zwrk.go·2                             1.14M ± 4%
	fs1-zwrk.go·3                             1.60M ± 2%
	fs1-zwrk.go·4                             2.09M ± 1%
	fs1-zwrk.go·8                             2.74M ± 1%
	fs1-zwrk.go·12                            2.76M ± 0%
	fs1-zwrk.go·16                            2.76M ± 1%
	zeo/py/fs1-zwrk.go·1                      9.42k ± 9%
	zeo/py/fs1-zwrk.go·2                      10.4k ± 1%
	zeo/py/fs1-zwrk.go·3                      11.4k ± 1%
	zeo/py/fs1-zwrk.go·4                      11.7k ± 2%
	zeo/py/fs1-zwrk.go·8                      12.4k ± 1%
	zeo/py/fs1-zwrk.go·12                     12.5k ± 1%
	zeo/py/fs1-zwrk.go·16                     13.4k ±11%

	name                             latency-time/object
	cluster:rio dataset:wczblk1-8
	fs1-zwrk.go·1                            2.63µs ± 2%
	fs1-zwrk.go·2                            3.00µs ± 3%
	fs1-zwrk.go·3                            3.16µs ± 1%
	fs1-zwrk.go·4                            3.23µs ± 1%
	fs1-zwrk.go·8                            4.94µs ± 0%
	fs1-zwrk.go·12                           7.06µs ± 0%
	fs1-zwrk.go·16                           9.36µs ± 0%
	zeo/py/fs1-zwrk.go·1                      121µs ± 1%
	zeo/py/fs1-zwrk.go·2                      192µs ± 2%
	zeo/py/fs1-zwrk.go·3                      267µs ± 1%
	zeo/py/fs1-zwrk.go·4                      343µs ± 1%
	zeo/py/fs1-zwrk.go·8                      660µs ± 2%
	zeo/py/fs1-zwrk.go·12                     977µs ± 1%
	zeo/py/fs1-zwrk.go·16                    1.30ms ± 2%
	cluster:rio dataset:prod1-1024
	fs1-zwrk.go·1                            1.69µs ± 7%
	fs1-zwrk.go·2                            1.76µs ± 4%
	fs1-zwrk.go·3                            1.88µs ± 2%
	fs1-zwrk.go·4                            1.91µs ± 1%
	fs1-zwrk.go·8                            2.92µs ± 1%
	fs1-zwrk.go·12                           4.34µs ± 0%
	fs1-zwrk.go·16                           5.80µs ± 1%
	zeo/py/fs1-zwrk.go·1                      107µs ± 9%
	zeo/py/fs1-zwrk.go·2                      192µs ± 1%
	zeo/py/fs1-zwrk.go·3                      263µs ± 1%
	zeo/py/fs1-zwrk.go·4                      342µs ± 2%
	zeo/py/fs1-zwrk.go·8                      648µs ± 1%
	zeo/py/fs1-zwrk.go·12                     957µs ± 1%
	zeo/py/fs1-zwrk.go·16                    1.20ms ±10%

The scalability graphs in http://navytux.spb.ru/~kirr/neo.html were
made with simulating client load by zwrk, not many client OS processes.
http://navytux.spb.ru/~kirr/neo.html#performance-tests has some
additional notes on zwrk.

Some draft history related to this patch:

	lab.nexedi.com/kirr/neo/commit/ca0d828b	X neotest: Tzwrk1 - place to control running time of 1 zwrk iteration
	lab.nexedi.com/kirr/neo/commit/bbfb5006	X zwrk: Make sure we warm up connections to all NEO storages when cluster is partitioned
	lab.nexedi.com/kirr/neo/commit/7f22bba6	X zwrk: New tool to simulate paralell load from multiple clients