Replacing pthread_mutex with std::mutex on MDNS

Use standard library provided lock and mutex. Also, add Clang thread
annotations on MDNS.

Test: runtests.sh pass
Change-Id: I220db2654ec0a36e6353614cd5e6cf2b229a4b0b
Merged-In: I220db2654ec0a36e6353614cd5e6cf2b229a4b0b
2 files changed