feat(core): maintain transaction context automatically

[B4nan]

Domain API is now used behind the scenes to automatically bare the tx context. This means
we can now use any EM, repository or a service (from DI), regardless of what EM instance
they were created from.

// this repo is based on `orm.em`, but thanks `TransactionContext` helper,
// it will use the right EM behind the scenes when used inside `em.transactional()`
const repo = orm.em.getRepository(Author4);

await orm.em.transactional(async () => {
  const a = await repo.findOneOrFail(god1);
  a.name = 'abc';
});

[lgtm-com]

This pull request introduces 1 alert when merging bf38e7c into 7248762 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[merceyz]

This will override the current domain.active meaning any custom things in there will be lost when using transactional, could it instead attach itself to the current active domain or copy the data from the active domain?

import domain from 'domain';

(async () => {
	const userDomain = domain.create();
	userDomain.__requestId = 1;

	userDomain.run(async () => {
		console.log(domain.active.__requestId);
		// 1

		const transactionalDomain = domain.create();
		transactionalDomain.__transactional = 2;
		transactionalDomain.run(async () => {
			console.log(domain.active.__requestId, domain.active.__transactional);
			// undefined, 2
		});
	});
})();

[B4nan]

Sure, good idea