reducing rxbuffers to the max RX payload size;
All copies to these buffers occur in the priority 5 UART RX
interrupts.  These interrupts block further priority 5
interrupts.  Thus, it is not possible to have more than
one payload stored in these buffers and the size may
be reduced to MAX_RX_PAYLOAD_SIZE.

Note that this is NOT the case for the tx buffers.  These
buffers must have room for both escape characters AND multiple
payloads at once.