mdbx: more yields as workaround for pthreads bug.
mdbx: more yields as workaround for pthreads bug.

Backport 719ecd55cf1d0ea7d6771248b3936f056bc49a47 from https://github.com/ReOpen/ReOpenLDAP

Change-Id: I6bead57782a29fca5352055db6afee02ad1ee38b

leo-yuriev/libfpta,leo-yuriev/libfpta