[dpl] support yieldable log processing

[cmattatall]

There are many scenarios where the core performing log processing is also performing other I/O bound workloads and the log processing is not the highest priority task.

This PR preserves the default behavior of DebugP_shm_log_reader (reading all available logs, blocking until all are processed) but exposes an API that allows yielding and after a certain number of lines have been processed.