Fix spamming of key-share-requests
Fix spamming of key-share-requests

Fixes a race in the memory-backed crypto store which meant that we would spam
out multiple key-share-requests for the same session.

(This didn't happen very often in practice, because normally we use the
indexeddb-backed store, which is race-free. Or at least, doesn't have this
race.)

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