Fix: Multicast pipe0 address handling issue

Previous behaviour:
1. Radio #1: Pipe 0 opened for writing (Address1), pipe 1 opened for
reading (Address2)
2. The radio would end up listening on the following pipes/addresses,
because pipe0 was not assigned a separate reading address.
Pipe0: Address1
Pipe1: Address2
3. This is generally not a problem with 2 radios, but when multicasting
with three. If two of the radios transmit to the same address, then
start listening, all three radios will be listening to the same address
on pipe0, unless pipe0 has been assigned a separate reading address

New behaviour:
1. Radio #1: Pipe 0 opened for writing (Address1), pipe 1 opened for
reading (Address2)
2. When calling radio.startListening() pipe0 is closed, because it is
not assigned a reading address
3. Pipe0 is re-opened for writing only, unless a reading address is
assigned to pipe0

This wouldn't really affect things while using auto-ack, because two
radios should not be writing to the same pipe/address.

- Add closeReadingPipe for RPi
- Also adjusted timing for startListening();
- Fix failure detect variable on RPi