Unify logging to only use Flux logging

[ilumsden]

Right now, we have 3 mechanisms to log information in DYAD:

• DYAD_LOG_INFO, DYAD_LOG_ERR, FLUX_LOG_INFO, FLUX_LOG_ERR (i.e., Flux logging)
• DPRINTF
• IPRINTF

We should unify these approaches together into a single approach. I'd recommend that we use the first approach (i.e., DYAD_LOG_INFO, etc.) for this.

[JaeseungYeom]

We are almost there.
We currently support flux based logging, cpp-logger library based logging, and stdout/stderr.
However, the last one is folded into Flux-based logging. This should be fixed.
The last case has unique advantage where it does not rely on any other infrastructure. So, it is easier for unit testing, and utility code.

[wangvsa]

Do we want to keep all three loggers: FLUX, CPP and PRINTF?
Currently, setting "-DDYAD_LOGGER=PRINTF" will cause compile errors.