Adds a new setting `minSendBytes` that is `undefined` by default.

If `undefined` all messages sent through multiplexed streams will
be serialized and sent over the wire immediately.

If set to a number, it will be used as a byte value, and the
serialized bytes of all messages sent during the current tick will
be buffered up to that value.

Once either the buffer lengths hits that value or the next tick
begins, all bytes in the buffer will be sent over the wire.

…te-batching