js: reduce request spam at getKeys()

• Don't make a secret-Key request for each Key retrieved, use one request for all of them instead, and assemble the info later. This should reduce the traffic with large Keyrings. The bulk retrieval for the public armored Keys for each of these Keys is still up to discussion

• unittests: disabled assertion for the armored key (as it currently doesn't work)

• encryptTest: clarified the mechanism/reason of rejection for Messages >64 MB. This is still a TODO, as this error comes from a different place (the browser itself) and behaves different from the other errors.