SERVER-13600 Forbid implicit construction of NamespaceString from StringData.
SERVER-13600 Remove string-copying implicit conversion operator from NamespaceString to std::string.

By replacing the implicit conversion operator from const
NamespaceString&->std::string with one from const NamespaceString&->const
std::string&, we at least eliminate an implicit copy.  Better might be to get
rid of the implicit conversion entirely, but we might want to start by removing
APIs that take std::string when they mean NamespaceString.