Inexisting keys are logged on default configuration

[jirfag]

Found during benchmarking tarantool-memcached vs memcached, that tarantool-memcached eats 100% of CPU on logging:

...
2016-04-24 08:31:02.782 [29594] main/982/server/52.58.16.52:58199 proto_binary.c:988 E> memcached error 1: Not found
2016-04-24 08:31:03.030 [29594] main/983/server/52.58.16.52:58200 proto_binary.c:988 E> memcached error 1: Not found
...

log_level is default, 5. I think we need log it on debug level.

[bigbes]

Fixed in latest master. Now it depends on log level