Simplify db_copy_mgr_t::add_value()

The reason we had a different implementation for "double" values here is
that std::to_string() uses the current locale for writing out numbers.
For integers this doesn't matter, but for doubles this could be
something different than a decimal point. The fmt::to_string() function
on the other hand always uses the default locale, so it doesn't have
this problem. So we can use this for integers as well as for doubles.

Author: joto

src/db-copy-mgr.hpp

@@ -290,13 +290,7 @@ class db_copy_mgr_t
     template <typename T>
     void add_value(T value)
     {
-        m_current->buffer += std::to_string(value);
-    }
-
-    void add_value(double value)
-    {
-        util::double_to_buffer tmp{value};
-        m_current->buffer += tmp.c_str();
+        m_current->buffer += fmt::to_string(value);
     }
 
     void add_value(std::string const &s) { add_value(s.c_str()); }