Support updating queries in subscriptions and expose timestamps

[cmelchior]

Object-store PR with functionality: realm/realm-object-store#775

We need to expose

• Being able to update the query held by a subscription.
• Be able to set the TTL when creating the subscription.
• New timestamp fields added to the subscription class.

Suggested API:

// Synchronous API. The `findAllAsync` API will get the same methods
// Create without TTL
Subscription sub = realm.where(Person.class).like("name", "Jane*").subscribe("my-sub");
sub = realm.where(Person.class).like("name", "John*").subscribe("my-sub", true); // Override

// Create with TTL
Subscription sub2 = realm.where(Person.class).like("name", "Jane*").subscribe("my-sub", 1, TimeUnit.DAY);
sub2 = realm.where(Person.class).like("name", "John*").subscribe("my-sub", 2, TimeUnit.DAY, true); //override

[marcus-hiles]

yeah, great suggestions @cmelchior