sleeping for longer period (> 15 minutes) can cause delayed transmission about arduino-lmic HOT 1 OPEN

The quickfix above has the drawback, that it allows immediate transmission after sleep. For the case, that the avail time is intentionally more in the future, this would lead to a too-early transmit. I propose a more complete solution, which consists of two steps: 1. after sleep, add the sleep time to the timer, to keep track of the real elapsed time. This was proposed in https://www.thethingsnetwork.org/forum/t/lmic-plus-rfm95w-staying-awake-for-long-periods/7506/44
The hacky version for 8 seconds arduino-sleep:
//Give the AVR back the slept time back (simple version)
cli();
timer0_overflow_count += 8 * 64 * clockCyclesPerMicrosecond(); //give back 8 seconds of sleep
sei();
os_getTime(); /* VERY IMPORTANT after sleep to update os_time and not cause txbeg and
os_time out of sync which causes send delays with the RFM95 on and eating power */

Step 2: Make sure, that the avail-variables do not run too much into the past. Pseudocode: if avail is in the past, then avail = now. I referenced my bugfix proposal in #934.

from arduino-lmic.