Use of string instead of std::string in some source files prevents compilation

If you build with --use-system-pcre with version 8.41 of pcre MongoDB fails to build because string is undefined. Replacing string with std::string in those files allows it to build again.
This applies to:
src/mongo/db/matcher/expression_leaf.cpp
src/mongo/db/repl/master_slave.cpp
src/mongo/shell/bench.cpp

This is preventing it building on gentoo linux.