Add SQLite support, fix un-thread-safe DB caches, small performance f… (#172)

* Add SQLite support, fix un-thread-safe DB caches, small performance fixes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add SQLite licenses to README

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* appease the linter, and fix my dumbass-ery

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* make requested changes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add back comment

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

vendor/modernc.org/mathutil/permute.go

@@ -0,0 +1,39 @@
+// Copyright (c) 2014 The mathutil Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mathutil // import "modernc.org/mathutil"
+
+import (
+	"sort"
+)
+
+// PermutationFirst generates the first permutation of data.
+func PermutationFirst(data sort.Interface) {
+	sort.Sort(data)
+}
+
+// PermutationNext generates the next permutation of data if possible and
+// return true.  Return false if there is no more permutation left.  Based on
+// the algorithm described here:
+// http://en.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order
+func PermutationNext(data sort.Interface) bool {
+	var k, l int
+	for k = data.Len() - 2; ; k-- { // 1.
+		if k < 0 {
+			return false
+		}
+
+		if data.Less(k, k+1) {
+			break
+		}
+	}
+	for l = data.Len() - 1; !data.Less(k, l); l-- { // 2.
+	}
+	data.Swap(k, l)                             // 3.
+	for i, j := k+1, data.Len()-1; i < j; i++ { // 4.
+		data.Swap(i, j)
+		j--
+	}
+	return true
+}