Insights: expired events in a batch make the server decline the whole batch

[VladislavFitz]

Describe the bug 🐛

The server doesn't accept the Insights events older than 4 days.
If a batch contains "expired" events and fresh events, they will all be declined by the server.

To Reproduce 🔍

Steps to reproduce the behavior:

1. Track a few Insights events assigning them more than 4 days old timestamps
2. Track a few Insights events with real-time timestamps
3. Flush the events
4. The server will decline the events batch

Expected behavior 💭

Before sending the events batch to the server, the expired events must be excluded.