prevent overlapping sync accumulator persists
prevent overlapping sync accumulator persists

add a flag to stop the sync worker trying to persist to indexeddb
if there are already persists in flight. accumulates user presence
updates in RAM to stop them being lost if the persist is skipped.

hopefully fixes https://github.com/vector-im/element-web/issues/21541

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk