## Problem
We only trim the senders if we tried to send a message to them and
discovered that the channel is closed. This is problematic if the
pageserver keeps connecting while there's nothing to send back for the
shard. In this scenario we never trim down the senders list and can
panic due to the u8 limit.
## Summary of Changes
Trim down the dead senders before adding a new one.
Closes LKB-178
c848b995b2