C++11 cleanup for util/mutexlock.h.
C++11 cleanup for util/mutexlock.h.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213398583

likema/leveldb,dwing4g/leveldb,likema/leveldb,fastogt/leveldb,bitcoin-core/leveldb,matehat/leveldb,dwing4g/leveldb,dwing4g/leveldb,bitcoin-core/leveldb,matehat/leveldb,bitcoin-core/leveldb,google/leveldb,fastogt/leveldb,google/leveldb,matehat/leveldb,cmumford/leveldb,cmumford/leveldb