In this commit, we start a timer if a mailbox stream is completely
un-occupied (neither read or write stream is occupied). The timer
stopped if either of the streams are occupied and is reset if both
streams are unoccupied.
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>