sdlog2 optimization

Use orb_check() instead of poll(), my expirience says that it has a bit less overhead. Now it's impossible to log at full rate, but in real system it's anyway impossible because of large CPU load. I will be happy if it will free 2-3% of CPU.

A lot of code stripped, adding new streams will be a bit simpler.

Some compiler warnings and minor bugs fixed.

TODO
 - Smoothly increase sleep delay when log buffer becomes full (e.g. > 75%). It's better to have some samples with low rate instead hole in log.

Not tested yet!