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/golang.org/x/xerrors/wrap.go

@@ -0,0 +1,106 @@
+// Copyright 2018 The Go 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 xerrors
+
+import (
+	"reflect"
+)
+
+// A Wrapper provides context around another error.
+type Wrapper interface {
+	// Unwrap returns the next error in the error chain.
+	// If there is no next error, Unwrap returns nil.
+	Unwrap() error
+}
+
+// Opaque returns an error with the same error formatting as err
+// but that does not match err and cannot be unwrapped.
+func Opaque(err error) error {
+	return noWrapper{err}
+}
+
+type noWrapper struct {
+	error
+}
+
+func (e noWrapper) FormatError(p Printer) (next error) {
+	if f, ok := e.error.(Formatter); ok {
+		return f.FormatError(p)
+	}
+	p.Print(e.error)
+	return nil
+}
+
+// Unwrap returns the result of calling the Unwrap method on err, if err implements
+// Unwrap. Otherwise, Unwrap returns nil.
+func Unwrap(err error) error {
+	u, ok := err.(Wrapper)
+	if !ok {
+		return nil
+	}
+	return u.Unwrap()
+}
+
+// Is reports whether any error in err's chain matches target.
+//
+// An error is considered to match a target if it is equal to that target or if
+// it implements a method Is(error) bool such that Is(target) returns true.
+func Is(err, target error) bool {
+	if target == nil {
+		return err == target
+	}
+
+	isComparable := reflect.TypeOf(target).Comparable()
+	for {
+		if isComparable && err == target {
+			return true
+		}
+		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
+			return true
+		}
+		// TODO: consider supporing target.Is(err). This would allow
+		// user-definable predicates, but also may allow for coping with sloppy
+		// APIs, thereby making it easier to get away with them.
+		if err = Unwrap(err); err == nil {
+			return false
+		}
+	}
+}
+
+// As finds the first error in err's chain that matches the type to which target
+// points, and if so, sets the target to its value and returns true. An error
+// matches a type if it is assignable to the target type, or if it has a method
+// As(interface{}) bool such that As(target) returns true. As will panic if target
+// is not a non-nil pointer to a type which implements error or is of interface type.
+//
+// The As method should set the target to its value and return true if err
+// matches the type to which target points.
+func As(err error, target interface{}) bool {
+	if target == nil {
+		panic("errors: target cannot be nil")
+	}
+	val := reflect.ValueOf(target)
+	typ := val.Type()
+	if typ.Kind() != reflect.Ptr || val.IsNil() {
+		panic("errors: target must be a non-nil pointer")
+	}
+	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
+		panic("errors: *target must be interface or implement error")
+	}
+	targetType := typ.Elem()
+	for err != nil {
+		if reflect.TypeOf(err).AssignableTo(targetType) {
+			val.Elem().Set(reflect.ValueOf(err))
+			return true
+		}
+		if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
+			return true
+		}
+		err = Unwrap(err)
+	}
+	return false
+}
+
+var errorType = reflect.TypeOf((*error)(nil)).Elem()