Chats pruning (one-to-one and groups) #640

In order to keep the backup of an account short and light, but also to have shorter recovery times (think about groups or those very long chats) I ask that it be introduced the possibility of setting a length limit for the individual chats and the groups.

This limit should not have a time limit, but a unitary one.

For example: Always keep the last 100 messages.

This limit is set locally, on the client device, so the server (in the case of groups) will only deliver the client messages that meet its local rule (which is a user-decided value).

This is something we discussed in the very early days of Cwtch, and I believe an early prototype might have even had support for it but it never migrated past alpha.

Ideally I think we would allow both time-based pruning (save messages for 1 day / week / month), and absolute pruning (100 max messages).

There is already a prune messages api in cwtch that checks the save history status. It would not be too much work to have it check additional qualifiers.

Related to: #608