exceptions (return False from __exit__) in _openlock context manager

utils and make them usable in a more general context

level to INFO.
one exception: left one log with level info in planning in show_resources
which is explicitely printing.

allows comparing HashableDict to over objects, and allows finding equal a
HashableDict and a dict with the same content.

[execo_engine] ParamSweeper: fix fopen mode a+ does not read at beginning on darwin (and bsd probably)

needed for filtering efficiently

skipped / inprogress / done for computing stats

bugfix: inprogress not correctly read (wrong filepos) in full_update
optim: inprogress not loaded 2 times when incremental update fails and
reverts to full_update

because if returning the real class member, client code can easily run in
situations where it uses the iterable returned (eg. iterating over it)
while it is concurrently modified by another thread

in particular: errors that may arise when killing a ParamSweeper when it
is writing to disk. inprogress will be lost, but only that last written
done will be lost. And future run will self heal the files (truncate the
buggy part)

done, remaining, inprogress, skipped are now consistenly maintained, not
recomputed continuously
*important* storage format of done changes: it was a pickled set, it is
now the concatenation of individual pickled combinations