This patch adds a config option that enables a mode wher a proxy will
only keep the latest room key from a sender in a certain room. This is
useful for bots that use pantalaimon since they are mostly only
interested in the latest messages and don't care about room history.
Since some public rooms can be quite large downloading the room history
for such rooms can be quite resource intensive.
Replicating the whole room history locally for such rooms might be
undesirable. This patch adds the ability to only download room history
and index messages for encrypted rooms.
Do note that the search API supports searching in multiple rooms, thus
if the indexing is restricted to encrypted rooms a search across
multiple rooms will need to make a search request to the server as well
as a local search. This mode is currently unsupported.
