[spi-hdlc-adapter] enhance the retry logic when slave is busy (#2647)
[spi-hdlc-adapter] enhance the retry logic when slave is busy (#2647)

This commit changes the `spi-hdlc-adapter` retry logic when the slave
is busy (it refuses a transmission). There are three levels, initially
the retries happen almost immediately, then up to certain number of
attempts at a fast rate and finally it drops to a slow rate.

This change helps with the average delay of spinel message exchange
between NCP and host (connected using an SPI interface).
abtink/openthread,bukepo/openthread,chshu/openthread,mszczodrak/openthread,lanyuwen/openthread,pvanhorn/openthread,lanyuwen/openthread,abtink/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,chshu/openthread,turon/openthread,turon/openthread,srickardti/openthread,gandreello/openthread,lanyuwen/openthread,georgecpr/openthread,pvanhorn/openthread,abtink/openthread,erja-gp/openthread,lanyuwen/openthread,LeZhang2016/openthread,openthread/openthread,georgecpr/openthread,erja-gp/openthread,gandreello/openthread,erja-gp/openthread,pvanhorn/openthread,chshu/openthread,georgecpr/openthread,librasungirl/openthread,gandreello/openthread,LeZhang2016/openthread,georgecpr/openthread,gandreello/openthread,openthread/openthread,mszczodrak/openthread,mszczodrak/openthread,jwhui/openthread,mszczodrak/openthread,abtink/openthread,pvanhorn/openthread,erja-gp/openthread,LeZhang2016/openthread,georgecpr/openthread,gandreello/openthread,LeZhang2016/openthread,mszczodrak/openthread,srickardti/openthread,jwhui/openthread,bukepo/openthread,LeZhang2016/openthread,openthread/openthread,pvanhorn/openthread,gandreello/openthread,bukepo/openthread,jwhui/openthread,bukepo/openthread,chshu/openthread,LeZhang2016/openthread,mszczodrak/openthread,georgecpr/openthread,turon/openthread,erja-gp/openthread,pvanhorn/openthread,turon/openthread,srickardti/openthread,chshu/openthread,librasungirl/openthread,erja-gp/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread