[web] Update serverID for pendingUploads after reassignment

Summary:
Follow up https://phab.comm.dev/D13501?id=44638#inline-77322

Depends on D13555

Test Plan: Printed pendingUploads in setState callback. Started thread with a user without CSAT by sending multimedia. Confirmed that upload serverID is updated to a keyserver ID.

Reviewers: kamil, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13556