Refactor regex code

This commit fixes quite a few odd things in regex code:
 * Lack of encapsulation.
 * Non-method functions for matching without retrieving all groups.
 * Regex class being copyable without proper copy-constructor (potential UAF
   and double free due to pointer members m_pc and m_pce).
 * Redundant SMatch::m_length, which always equals to match.size() anyway.
 * Weird SMatch::size_ member which is initialized only by one of the three matching
   functions, and equals to the return value of that function anyways.
 * Several places in code having std::string value instead of reference.

src/collection/backend/in_memory-per_process.cc

@@ -134,7 +134,7 @@ void InMemoryPerProcess::resolveRegularExpression(const std::string& var,
     //std::string name = std::string(var, var.find(":") + 2,
     //    var.size() - var.find(":") - 3);
     //size_t keySize = col.size();
-    Utils::Regex r = Utils::Regex(var);
+    Utils::Regex r(var);
 
     for (const auto& x : *this) {
         //if (x.first.size() <= keySize + 1) {