E2E Key Backup: Recovery key is a pickle of the Olm PkEncryption object which is several time longer than it needs to be

[lampholder]

Spilt out from #3661

[dbkr]

https://github.com/matrix-org/olm-backup/pull/61 exposes the appropriate functionality in Olm, rest of this is in the various dbkr/e2e_backups branches

[jryans]

(I am assuming this is fixed because it was marked as In Test. @dbkr, please reopen if incorrect.)