Limiting size of outgoing messages while coalescing parcels

This PR adds code which limits coalescing parcels into messages based on a maximum message size. This value is configurable either at compile time (`HPX_PARCEL_MAX_OUTBOUND_MESSAGE_SIZE`) or at runtime (`hpx.parcel.max_outbound_message_size`). The current default value is `1000000` bytes.
Not sure what this change is about, but IBVERBS can't be used for bootstrapping