fix for realtime calling subscription multiple time - Appwrite 1.4.x

[lohanidamodar]

What does this PR do?

• Fixes issue with calling subscribe multiple times
• Not syncing with main so this remains, if we want to release Flutter SDK with this fix for Appwrite 1.4.x
• For latest PR check: fix for realtime multiple subscription 15x #801
• We shouldn't merge this, rather use this if we want to release (as main has few new features I think not suitable for generating with spec for 1.4.x) if not we can close this issue

Test Plan

• Manually testing the generated SDK

Related PRs and Issues

• 🐛 Bug Report: Subscribing Multiple Times with a Single Realtime Instance Results in Null Subscription sdk-for-flutter#156

Have you read the Contributing Guidelines on issues?

YES

[stnguyen90]

I think using a counter can lead to problems. For example:

0. start (counter = 0, subscriptions map is empty)
1. subscribe() (counter = 1, subscriptions[0] is set)
2. subscribe() (counter = 2, subscriptions[0] and [1] are set)
3. the first one gets removed (counter = 1, subscriptions[1] is set)
4. a new subscription added (counter = 2, subscriptions[1] is set and replaces the subscription from step 2)

[lohanidamodar]

Thanks, I've updated to use the time in microseconds to overcome this. please let me know what you think.