les binary search
for supported keys. The lookup also reduces the io frequency, so the
original mutex is replaced by locks around the actual io.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
[guenter.roeck@ericsson.com: Added value range check to key_at_index_store()]
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
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