[util] switch to using std::shared_lock

Since Kudu switched to using C++17 long time ago, the home-grown
kudu::shared_lock is no longer needed.

Change-Id: Idcda724a3bdab86f593d0241e062dd40bcd26af1
Reviewed-on: http://gerrit.cloudera.org:8080/21349
Reviewed-by: Wang Xixu <1450306854@qq.com>
Tested-by: Alexey Serbin <alexey@apache.org>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
42 files changed