fix #61: Don't respond to bogus frames

[dgoo2308]

as per @IanAber (#61) prevent sending STATUS_ILLEGAL_FUNCTION on a bogus frame:

• validateRequest(): move CRC and validate address to start of procedure

Some extras for extra snappy performance optimalisations:

• relevantAddress(): keep the check it local in , since we provide the unitAddress to check for broadcast.
• validateRequest(): keep is_Broadcast check local , it's all here i.s.o. calling isBroadcast() => calling readUnitAddress() : inline with createResponse()
• executeCallback: remove bool isbroadcast and avoid thus extra branch check and memory usage

[yaacov]

:green_heart:

[falahati]

Calculating CRC is costly; should have it after every other validation that might narrow the execution cost down. That was why we put it down in the execution path after address check and length check. Especially on a busy network, it will push every device to the limit.

[falahati]

Maybe just store the STATUS_ILLEGAL_FUNCTION in a variable and send it back if the CRC is also correct? This allows the device to skip CRC check for valid or invalid requests not targetted toward it and fixes the issue in question without adding additional performance cost to the function.

[dgoo2308]

@falahati noted, will look into that possibility, I see now the reason behind it, thanks!