extend message lifetime when space allows

[mwfarb]

Currently all messages are scrubbed from the server after 24 hours. This make message availability confusing for people who are offline more than 24 hours, since they will receive the push notice, but know nothing about the actual message since the ciphertext has been removed from the server and cannot be downloaded.

Solution for Adequate Extra Space in Push Notice:

• We can add the cipertext in a new JSON pair in the message.
• Our server message deleted at 24 hours, but Push Service provide may be able to deliver.

Solution for Not Enough Extra Space in Push Notice:

• We send a push message revoke notice to the Push Service at the delete mark.
• Our server message deleted at 24 hours, user would not be annoyed with expired notice when user comes online.

[mwfarb]

push message packet size is way too small for iOS versions (256 bytes) for minimum ciphertext size, so better to allow some to remain longer on the server.