env.sync() blocks the micro event queue (promise callbacks) in Node when other timeouts are in the queue

[kriszyp]

This is a minimal test case to reproduce the issue:

const { Env } = require('node-lmdb');
let env = new Env();
env.open({path: 'test'}); // some path for the db
new Promise(resolve => {
	env.sync(() => {
		console.log('synced');
		resolve();
	})
}).then(() => {
	// this should execute immediately after it is synced
	console.log('then');
});
setTimeout(() => {
	console.log('longer timeout');
}, 10000);

Expected output:

synced
then
longer timeout

Actual output:

synced
longer timeout
then