Let lock_guard deduce its template argument

No functional change, this is a cleanup.

With C++17, it's no longer necessary to specify the teplate argument
when it can be deduced from the types of constructor arguments. This
allows de-cluttering our locking statements.

To avoid typos, this patch was mechanically generated:

  perl -p -i -e 's/std::lock_guard<std::mutex>/std::lock_guard/g' \
    $(find . -name '*.cpp' -o -name '*.h')

Change-Id: Ibb15d9a6c5b1c861d81353e47d25474eb1d4c2df
diff --git a/server/dns/DnsTlsDispatcher.cpp b/server/dns/DnsTlsDispatcher.cpp
index 95fbb9a..c336f54 100644
--- a/server/dns/DnsTlsDispatcher.cpp
+++ b/server/dns/DnsTlsDispatcher.cpp
@@ -44,7 +44,7 @@
     // Pull out any servers for which we might have existing connections and
     // place them at the from the list of servers to try.
     {
-        std::lock_guard<std::mutex> guard(sLock);
+        std::lock_guard guard(sLock);
 
         for (const auto& tlsServer : tlsServers) {
             const Key key = std::make_pair(mark, tlsServer);
@@ -113,7 +113,7 @@
     const Key key = std::make_pair(mark, server);
     Transport* xport;
     {
-        std::lock_guard<std::mutex> guard(sLock);
+        std::lock_guard guard(sLock);
         auto it = mStore.find(key);
         if (it == mStore.end()) {
             xport = new Transport(server, mark, mFactory.get());
@@ -144,7 +144,7 @@
 
     auto now = std::chrono::steady_clock::now();
     {
-        std::lock_guard<std::mutex> guard(sLock);
+        std::lock_guard guard(sLock);
         --xport->useCount;
         xport->lastUsed = now;
         cleanup(now);