Add user TTL refresh

[data-sync-user]

Bigtable requires a cell timestamp and family, which is used for automatic garbage collection for data. (Once all cells for a given key are removed, the key is also removed.) Each family can specify when data should be deleted. A cell family can have a TTL rule added which will take effect immediately on all existing cell timestamps.
We currently are writing cell timestamps for all cells, but have not activated automatic deletion for all families. This means that we may retain some data longer than necessary.

Mobile devices will check in once a day, but we can also use message hand-off to the bridge as a reasonable proxy for device “liveliness” (See https://docs.google.com/document/d/1M_zJ9P3LI_lfs1xrDapIY6OJMRwQPesXlxVpOK0fvPM/edit#heading=h.dwdigq1bm1w3 ). Desktop devices have far more active connections, but would still require timestamp refreshes.

We should consider creating some mechanism to update the associated cell timestamps once per 24 hours of activity for a given UAID. This would mean more writes per day (since we would be updating/duplicating cells) however this should not be a high cost.

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