SL-10291 Replace apr_mutex with standard C++11 functionality

doc/contributions.txt

indra/llcommon/llapr.cpp

indra/llcommon/llapr.h

indra/llcommon/llerror.cpp

indra/llcommon/llfixedbuffer.cpp

indra/llcommon/llmutex.cpp

indra/llcommon/llmutex.h

indra/llcommon/llthread.cpp

indra/llcommon/lluuid.cpp

indra/llcommon/llworkerthread.cpp

indra/llcorehttp/httpcommon.cpp

indra/llimage/llimage.cpp

indra/llimage/llimageworker.cpp

indra/llmath/llvolumemgr.cpp

indra/llmessage/llbuffer.cpp

indra/llmessage/llproxy.cpp

indra/llplugin/llpluginmessagepipe.cpp

indra/llplugin/llpluginprocessparent.cpp

indra/llvfs/llvfs.cpp

indra/newview/llappviewer.cpp