Network improvements

[2bndy5]

This ports changes from the following recent PRs to the pure python codebase:

• Fix for Multicast to level 4 RF24Network#227
• Enhance requestAddress by looping through poll responses RF24Mesh#238
• Enhance checkConnection() function RF24Mesh#240
• expose releaseAddress() for master node RF24Mesh#244
• Promiscuous scanners RF24#955
• mesh.checkConnection via parent not master RF24Mesh#250

[2bndy5]

I think the bug fixed in nRF24/RF24Network#229 is specific to the C++ implementation. Looking at the _write_to_pipe(), I forgot how much code gulf I played to speed things up.

There is no RF24Network.networkFlags here because

• message fragmentation is done directly in _write_to_pipe() (making FLAG_FAST_FRAG obsolete)
• FLAG_NO_PULL is implemented as _parenthood, even though there is no RF24Gateway/Ethernet implementation in this CirPy port

Furthermore, the network layers here don''t use a writefast() equivalent. Instead, I use a regular blocking write() (RF24.send()) and call RF24Network._tx_standby() which uses RF24.resend() similarly to how C++ RF24::txStandBy() uses RF24::reUseTx() but in a blocking manner.