Improve performance of VariableOrigin instances

- The previous approach would create a std::unique_ptr and store it in
  a std::list in VariableValue (Origins)
- The new approach now stores Origins in a std::vector and constructs
  VariableOrigin elements in-place on insertion.
- Instead of having two heap-allocations for every added VariableOrigin
  instance, this performs only one.
- If multiple origins are added, std::vector's growth strategy may even
  prevent a heap-allocation. There's a cost on growing the size of the
  vector, because a copy of current elements will be necessary.
  - Introduced reserveOrigin method to notify that multiple insertions
    will be made, so that we can use std::vector's reserve and do a
    single allocation (and copy of previous elements), and then just
    initialize the new elements in-place.

headers/modsecurity/anchored_variable.h

@@ -63,7 +63,7 @@ class AnchoredVariable {
     void append(const std::string &a, size_t offset,
         bool spaceSeparator = false);
     void append(const std::string &a, size_t offset,
-        bool spaceSeparator, int size);
+        bool spaceSeparator, size_t size);
 
     void evaluate(std::vector<const VariableValue *> *l);
     std::string *  evaluate();