In this commit, we add a mailbox-read-count metric. This will be
incremented each time a mailbox with an _odd_ stream ID is read from.
We do this because we assume that a full duplex connection is being used
meaning that there will be 2 streams that have a matching ID except for
the last byte. And so to avoid duplicating the data, we only record the
odd streams. We also assume that for every read, there will be a write
and so we only record the reads.
With this commit we fix a bug in the hashmail server that didn't return
a read stream properly if it was closed from the client side.
Co-authored-by: Elle Mouton <elle.mouton@gmail.com>
