[1.x] Improve subscription cancellation

[armiol]

This PR addresses #188.

In particular, the following changes were made to the behaviour:

• Calling unsubscribe for any subscription now leads to an immediate cancellation on both client- and server-side. Previously, all such cancellations were only processed during the next "keep-up" propagation.

• Client now allows cancelling all known subscription via cancelAllSubscriptions() no-args call. Its invocation leads to sending the bulk cancellation request to server-side, as well as shutting down the client-level subscriptions. Such an API is useful in case end-users choose to log out from the application.

Also, this PR aims to improve the server-side memory consumption.

In particular, server-side holds a couple of internal registries to track active subscriptions. Previously, upon cancellation of a subscription, some of these registries were not affected, thus producing a memory leak. Now, each cancelled subscription is removed from all in-memory caches.

In future PRs there will be more updates to this repository, addressing the library updates available for both Java and client-side code.

The library version is set to 1.9.0-SNAPSHOT.10.

[armiol]

@YegorUdovchenko @dmitrykuzmin since you are the original reporters, please have a look.

[armiol]

@YegorUdovchenko @dmitrykuzmin in scope of the following PRs, I am going to update the Firebase Admin SDK to the latest 9.1.x version. This will affect the Java API of web module as well. Please let me know if that's OK with you.

[YegorUdovchenko]

@YegorUdovchenko @dmitrykuzmin in scope of the following PRs, I am going to update the Firebase Admin SDK to the latest 9.1.x version. This will affect the Java API of web module as well. Please let me know if that's OK with you.

We'll have to version our server API during this migration anyway, so the Admin SDK upgrade is not a big deal.

[codecov[bot]]

Codecov Report

Merging #195 (f75bd3a) into v1 (95a0105) will decrease coverage by 0.78%. The diff coverage is 9.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v1 #195 +/- ## ============================================ - Coverage 63.17% 62.40% -0.78% - Complexity 214 215 +1 ============================================ Files 96 96 Lines 2724 2761 +37 Branches 46 47 +1 ============================================ + Hits 1721 1723 +2 - Misses 992 1027 +35 Partials 11 11 ```