Add distributed locking capability

[connor4312]

Example:

const { Etcd3 } = require('etcd3');
const client = new Etcd3();

client.lock('my_resource').do(() => {
  // The lock will automatically be released when this promise returns
  return doMyAtomicAction();
});

See the code for some more details. I branch off a bit from what, say python-etcd3 does and opted to use a lease for lock management. This prevents the resources from being locked indefinitely if an event occurs that prevents the consumer from unlocking the resource when they're finished with it. The logic is that the set of events which would allow a lease to be lost mid-lock is a subset of the events which would prevent the lease from ever being unlocked.

[SimonSchick]

Yo, if you target es6, you don't need this.

[connor4312]

Hm, I was still have some issues with the ts-node-based unit tests. I'll see if there's an option to get it to target ES6.

[SimonSchick]

Usually it uses es6 by default (because it's your tsconfig)

[connor4312]

It didn't for some reason, but setting an env var worked

[kragniz]

If you can wait until etcd-io/etcd#7444 is released, I'd recommend using that instead.

[connor4312]

Ah, neat, I wasn't aware that was a thing. I'll probably get this in and switch over to that at some point in the future -- should be possible to do without any breaking changes.

Addressed comments