Remove CoreConfig use of memtables (bug 5899 part 1, r=fyren).

public/amtl/am-string.h

@@ -45,6 +45,7 @@ class AString
    : length_(0)
   {
   }
+
   explicit AString(const char *str) {
     set(str, strlen(str));
   }
@@ -80,6 +81,12 @@ class AString
     }
     return *this;
   }
+  AString &operator =(Moveable<AString> other) {
+    chars_ = other->chars_.take();
+    length_ = other->length_;
+    other->length_ = 0;
+    return *this;
+  }
 
   int compare(const char *str) const {
     return strcmp(chars(), str);