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/libc/nopthreads.go

@@ -0,0 +1,33 @@
+// Copyright 2021 The Libc Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows darwin linux,arm linux,s390x
+
+package libc // import "modernc.org/libc"
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+var errno0 int32 // Temp errno for NewTLS
+
+type TLS struct {
+	ID                 int32
+	errnop             uintptr
+	reentryGuard       int32 // memgrind
+	stack              stackHeader
+	stackHeaderBalance int32
+}
+
+func NewTLS() *TLS {
+	id := atomic.AddInt32(&tid, 1)
+	t := &TLS{ID: id, errnop: uintptr(unsafe.Pointer(&errno0))}
+	if memgrind {
+		atomic.AddInt32(&tlsBalance, 1)
+	}
+	t.errnop = t.Alloc(int(unsafe.Sizeof(int32(0))))
+	*(*int32)(unsafe.Pointer(t.errnop)) = 0
+	return t
+}