[timer] adding TimerMilliContext class (#3364)
[timer] adding TimerMilliContext class (#3364)

This commit adds a new class `TimerMilliContext` which is a
millisecond timer that also maintains a user context pointer.

librasungirl/openthread,openthread/openthread,erja-gp/openthread,openthread/openthread,erja-gp/openthread,jwhui/openthread,abtink/openthread,bukepo/openthread,lanyuwen/openthread,abtink/openthread,abtink/openthread,jwhui/openthread,chshu/openthread,turon/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,jwhui/openthread,bukepo/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,abtink/openthread,librasungirl/openthread,bukepo/openthread,chshu/openthread,librasungirl/openthread,erja-gp/openthread,turon/openthread,srickardti/openthread,bukepo/openthread,erja-gp/openthread,librasungirl/openthread,srickardti/openthread,erja-gp/openthread,lanyuwen/openthread,lanyuwen/openthread,erja-gp/openthread,jwhui/openthread,srickardti/openthread,turon/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,turon/openthread,lanyuwen/openthread