Since timestamps are 64-bit, loosing one bit of range due to sign does
not present an issue, however for applications doing calculations on
timestamps, signed return type is more convenient.
The implementation of esp_timer_get_time used a critical section, which
resulted in a call time of ~1.8us. To make esp_timer_get_time more
useable as a high-resolution time source, this change replaces the lock
with polling. Call time is reduced to ~0.7us.
