Steve Howell 6fbaf7e80f Remove sender-related fields from message cache. ...

This change makes the cache entries smaller for message
dictionaries.  It also ensures we get valid data put into
message dictionaries if, for example, the sender's avatar
changes.

After this change, all of the attributes for a message
sender are only fetched during post-processing with two
exceptions:

    * We get sender_id for "free" from the message,
      and it's the primary key that we need to figure
      out which data to fetch in post-processing.

    * We need sender_realm_id to be able to cache topic
      links, and a sender's realm id will never change,
      so it's not a concern for invalidating cache rows.

All the other attributes are either likely to change (e.g.
sender avatar_version) and/or impact the size of cache
entries more severely than the two small id fields above.

This change should improve our overall system performance
by reducing the amount of memory used by every N message
rows we cache, and typically N will be in the thousands or
so on a large realm.

The other major implication of this change is that when
a user changes their avatar, and then later messages that
the user sent are fetched, all of the fields that go into
computing the avatar url will be pulled from the database,
not from cache.

2017-10-16 23:37:10 -07:00

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