common: add a class to safely wrap mlock/munlock

This class will allow mlocking small objects, of which there
may be several per page. It adds refcounting so pages are only
munlocked when the last object on that page munlocks.

tests/unit_tests/CMakeLists.txt

@@ -53,6 +53,7 @@ set(unit_tests_sources
   http.cpp
   main.cpp
   memwipe.cpp
+  mlocker.cpp
   mnemonics.cpp
   mul_div.cpp
   multisig.cpp