Honour keyshare requests from people you sent messages to.

[ara4n]

We apparently request keyshares from sending devices (as well as ourselves), but the sender ignores them like this:

2018-08-31T14:52:31.714Z I m.room_key_request from @Amandine:matrix.org:ZNPRXEQNQD for !teBWTcboSifjyWqxfY:matrix.org / f7mPJ4rZe3p8fuBpaOZL/SP1e6YQfXYtLlgP46mh2mU (id m1535727143583.162)
2018-08-31T14:52:31.714Z I Ignoring room key request from other user for now

If we actually upheld them, it'd work around a bunch of UISIs. If the sender can work out whether it's safely allowed to honour the req, it should.

[turt2live]

I believe the behaviour here has dramatically changed, and is pre-cross-signing.

Considering a duplicate of element-hq/element-meta#1888