Endless loop in SyncThread if network connection is lost during sync

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Describe the bug
When the network connection is lost during sync there's an endless loop in SyncThread. In this case the sync is retried until a connection is available again. See
https://github.com/vector-im/element-android/blob/1b9b18851d2389e41268a56229b8097e663c10b2/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/session/sync/job/SyncThread.kt#L129
and following lines.

This leads to an endless loop if the network connection is explicitly dropped or a network connection is not possible for other reasons for a longer time. This unnecessary increases the battery usage #536.

To Reproduce
Steps to reproduce the behavior:

1. Drop network connection during sync.

Expected behavior
The sync should immediately stop if the connection is dropped explicitly (if this can be checked through the Android API) and stop after a few retries. Maybe it makes sense to combine it with an increasing retry delay.

Smartphone (please complete the following information):

• Device: Samsung S5
• OS: Android 9 (Lineage OS 16.0)

Additional context

• develop branch at 1b9b188 with fdroid flavor
• Homeserver: matrix.org

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It might not be an issue when happening while Element is in foreground because SyncThread is paused when switching to background. Though it is an issue when happening during background sync. #1915 #1920