[XDCR] - Backfill mutations stuck when scopes and collections mapping is changed

Backfill mutations stuck when scopes and collections mapping is changed multiple times. The setting xdcrDevBackfillSendDelayMs was set to 1000 as well during the setup of the replication.

Steps to reproduce:

1. Setup 2 one node clusters
2. Create a test bucket in each cluster
3. Setup replication between the clusters
4. Pause replication
5. Change scopes and collections mapping
6. Resume replication
7. Repeat 4, 5 and 6 multiple times
8. Backfill mutations are stuck

Logs (Enterprise Edition 7.2.5 build 7578):
Source (172.23.105.4): https://cb-engineering.s3.amazonaws.com/MB-61210_src/collectinfo-2024-04-01T153443-ns_1%40cb.local.zip

Target (172.23.105.195): https://cb-engineering.s3.amazonaws.com/MB-61210_dest/collectinfo-2024-04-01T153503-ns_1%40cb.local.zip