Previously we were always testing with DBs backed up by FileStorage. Now
we provide a way to run the testsuite with user selected storage
backend:

    $ WENDELIN_CORE_TEST_DB="<fs>"   make test.py     # test with temporary db with FileStorage
    $ WENDELIN_CORE_TEST_DB="<zeo>"  make test.py     # ----------//---------- with ZEO
    $ WENDELIN_CORE_TEST_DB="<neo>"  make test.py     # ----------//---------- with NEO

    $ WENDELIN_CORE_TEST_DB=neo://db@master  make test.py     # test with externally provided DB

Default is still to run tests with FileStorage.

/cc @jm

Factor out those routines to open a ZODB database to common place.

The reason for doing so is that we'll soon teach dbopen to automatically
recognize several protocols, e.g. neo:// and zeo:// and this way,
clients who use dbopen() could automatically access storages besides
FileStorage.

    - for virtual memory subsytem
    - for ZBigFiles

They are not currently great, e.g. for virtmem we have in-kernel
overhead of page clearing - in perf profiles, for bigfile_mmap compared
to file_read kernel's clear_page_c raises significantly.

That is the worker for clearing page memory and we currently cannot
avoid that - any memory obtained from kernel (MAP_ANONYMOUS, mmap(file)
with hole, etc...) comes pre-initialized to zeros to userspace.

This can be seen in the benchmarks as well: file_readbig differs from
file_read in only that the latter uses 1 small buffer and the first
allocates large memory (cleared by kernel + python does the memset).

    bigfile/tests/bench_virtmem.py@125::bench_file_mmap_adler32     0.47  (0.86 0.49 0.47)
    bigfile/tests/bench_virtmem.py@126::bench_file_read_adler32     0.69  (1.11 0.71 0.69)
    bigfile/tests/bench_virtmem.py@127::bench_file_readbig_adler32  1.41  (1.70 1.42 1.41)
    bigfile/tests/bench_virtmem.py@128::bench_bigfile_mmap_adler32  1.42  (1.45 1.42 1.51)

    bigfile/tests/bench_virtmem.py@130::bench_file_mmap_md5         1.52  (1.91 1.54 1.52)
    bigfile/tests/bench_virtmem.py@131::bench_file_read_md5         1.73  (2.10 1.75 1.73)
    bigfile/tests/bench_virtmem.py@132::bench_file_readbig_md5      2.44  (2.73 2.46 2.44)
    bigfile/tests/bench_virtmem.py@133::bench_bigfile_mmap_md5      2.40  (2.48 2.40 2.53)

There is MAP_UNINITIALIZED which works only for non-mmu targets and only
if explicitly allowed when configuring kernel (off by default).

There were patches to disable that pages zeroing, as it gives
significant speedup for people's workloads, e.g. [1,2] but all of them
did not got merged for security reasons.

[1] http://marc.info/?t=132691315900001&r=1&w=2
[2] http://thread.gmane.org/gmane.linux.kernel/548926

~~~~

For ZBigFile - it is the storage who is dominating in profiles.