This is handy for remote characteristics because it allows for
the PacketBuffer binding code to have the correct max size. This
PR also adds checks so we don't write outside the outgoing buffer.
PacketBuffer facilitates packet oriented BLE protocols such as BLE
MIDI and the Apple Media Service.
This also adds PHY, MTU and connection event extension negotiation
to speed up data transfer when possible.
Scott Shawcroft
microDev
Dan Halbert