Reduced copying in cluster connectivity layer. A few TODOs still left...
Implemented message error handling. Closes #1914.
Avoid another copy pass when locally delivering messages.
A little refactoring of vector_read_stream_t
Added reserves and a tunable message read loop batch size.
directory_read_manager_t::on_message() didn't handle failed reads (e.g. due to disconnection).
Made exception handling in mailbox_t safer, and avoid one alloc in message handler.
Made the check for deserialization errors more explicit.
Merge branch 'daniel_cluster_msg_optimization' into next