Fix missing latest posts

[latter-bolden]

The prod build last night included a DB migration to save the new activity read marker setting. This uncovered a bug with our latest posts logic — when the migration wipes all SQL data, the keyval store keeps its existing headsSyncedAt timestamp. This means syncLatestPost thinks it already has messages that it doesn't.

To fix, we'll now clear out that key anytime we purge the DB. I tried upgrading from an older version to confirm it works, but wanted to double check with y'all if purge is always called in these migration scenarios?

Another thought I had: if we have more of these sync optimization flags, it might be worth considering an optional persistAfterMigration storage item flag (analogous to persistAfterLogout).

[patosullivan]

makes sense, let's just update electrondb as well before merging