pipeline sending of decryption keys and encrypting the decryption keys

Right now, `src/crypto/algorithms/megolm.js:_shareKeyWithDevices` splits devices into batches, and waits until one batch is completely finished before starting on the next batch.  Processing each batch consists of encrypting the key for each device, and then sending the keys.  We could save some time by starting the encryption for the next batch while the current batch is being sent.