fix(datastore): Remove unnecessary synchronized causing subscription slowness

[tylerjroach]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:

When DataStore is started, Create, Update, Delete subscriptions are made for each model. If I have 10 models, this results in 30 subscriptions established. Subscriptions are established over a single websocket connection by sending a websocket event per subscription. We wait until all subscriptions have been established (ws response message for each subscription) before we proceed with syncing/merging items.

An errant synchronized is causing subscription events to being be processed 1 at a time, and not in parallel.

If I have 10 models (30 subscriptions) and each ws event takes the server 200ms to acknowledge, establishing subscriptions would take the current build ~ 6 seconds. Allowing ws events in parallel will complete this effort in ~ 200ms.

Buggy State:
Send Model1 Create -> Model1 Create Response -> Send Model1 Delete -> Model1 Delete Response -> Send Model1 Update -> Model1 Update Response -> Send Model2 Create -> Model2 Create Response -> ...

Fixed State:
Send Model1 Create, Send Model1 Delete, Send Model1 Update, Send Model2 Create .... -> Wait for all acks

• The method is synchronized (problem): https://github.com/aws-amplify/amplify-android/blob/2145c0ac26ab9b406c36b299b3ab96[…]ain/java/com/amplifyframework/api/aws/SubscriptionEndpoint.java
• Inside the method, there is another synchronized lock (necessary) to only create 1 WS: https://github.com/aws-amplify/amplify-android/blob/2145c0ac26ab9b406c36b299b3ab96[…]ain/java/com/amplifyframework/api/aws/SubscriptionEndpoint.java
• Here is where we await inside the synchronized method for a single ws event response, causing the non-blocking ws.send methods to only send 1 event at a time: https://github.com/aws-amplify/amplify-android/blob/2145c0ac26ab9b406c36b299b3ab96[…]ain/java/com/amplifyframework/api/aws/SubscriptionEndpoint.java

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c839191) 42.86% compared to head (b06f4bd) 42.87%.

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Additional details and impacted files

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  Coverage   42.86%   42.87%           
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  Files         905      905           
  Lines       29098    29096    -2     
  Branches     4142     4142           
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+ Hits        12473    12475    +2     
+ Misses      15268    15265    -3     
+ Partials     1357     1356    -1     

[ankpshah]

Great find ! Thanks for fixing this.