AWS subscriptions - subscribe function is not thread safe and causes crashes

[Vanyaslav]

Describe the bug
When subscribing on multiple threads the connection pool is causing crashes since it accesses a dictionary inside the class IAMBasedConnectionPool which is not thread safe.

To Reproduce
Steps to reproduce the behavior:

1. Create multiple subscribers with AWS App sync on multiple threads.
2. Enjoy the crashing app.

Expected behavior
It should be a thread safe operation to avoid crashes.

Screenshots

Environment(please complete the following information):

• AppSync SDK Version: [3.6.2]
• Dependency Manager: [Cocoapods]
• Swift Version : [5.0]

Device Information (please complete the following information):

• Device: [Simulator]
• iOS Version: [e.g. iOS 16.4]
• Specific to simulators:

Additional context
We use the framework kinda extensively, but with iOS16.4 and AppSync v. 3.6.2 the problem has gone to really a bad situation, while before we had experienced the crash less frequently.
Already prepared a fix , gonna send a PR soon.

[atierian]

Resolved by #569