Add API for checking if time-critical jobs are pending

[terrillmoore]

When we're doing work in the background with the LMIC running in the foreground, we need a way to defer what we're doing to real-time future deadlines. Add an API,

bit_t os_queryTimeCriticalJobs(ostime_t time);

This API shall return true if any scheduled events should occur before time OS ticks have passed. This allows code to avoid starting a long computation if it would collide with a pending event for the LMIC.

[terrillmoore]

Committing as part of #215.