Makes regular expression selection on collections key case insensitive

This issue was initially reported by @michaelgranzow-avi on #2296.

@airween made an initial attempt to provide a fixed at #2107; As a
consequence of the pull request review - provided by @victorhora,
@zimmerle, and @michaelgranzow-avi - @airween made a second attempt
at #2297. After reviewing by @martinhsv, @zimmerle, I have absorbed
the essential pieces from @airween patch into this one.

This patch differs from @airween's because @airween's patches were
partially working: Key exclusions with regex weren't covered, same
for anchored variables (e.g. ARGS). During the review, I have
highlighted the importance of having elementary test cases. A simple
test case on ARGS could spot the issue. Since that is an important
fix, I don't want to hold this for one more review cycle; therefore,
I am committing the fix myself.

Thank you all involved in the solution of this very own issue.

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(var);
+    Utils::Regex r(var, true);
 
     for (const auto& x : *this) {
         //if (x.first.size() <= keySize + 1) {

src/collection/backend/lmdb.cc

@@ -537,7 +537,7 @@ void LMDB::resolveRegularExpression(const std::string& var,
     MDB_stat mst;
     MDB_cursor *cursor;
 
-    Utils::Regex r(var);
+    Utils::Regex r(var, true);
 
     rc = mdb_txn_begin(m_env, NULL, 0, &txn);
     lmdb_debug(rc, "txn", "resolveRegularExpression");