Add Milestone Request to message protocol

[kwek20]

Description

Requests a range of milestones by index. Expects to receive in response the milestone bundle for the specified index.

Motivation

Requesting milestones specifically will allow speed-up of syncing as we can do this in parallel instead of waiting until we find a new milestone by traversing down

Requirements

1. Add a new message type named MS_REQUEST in ProtocolMessage
2. Update MessageReader to read this message
3. Update NeighborImpl to handle the message

According to the following details: https://github.com/iotaledger/protocol-rfcs/blob/d16a3815271cdb4b53ff3c234ea706bf764e1847/text/0001-protocol-messages/0001-protocol-messages.md#milestone-request