Automatically determine best epilogue format

[ZwFink]

We are finding that the savings of diff-based epilogues is based on application behavior.
Generally, diff epilogues are better for more realistic workloads, where GPU kernels change a small portion of overall device memory.
Full epilogues are better for kernel benchmarks (e.g., HeCBench).
The differences are quite stark: up to 2x storage space increase for pathological diff cases, and up to 2x storage space increase for pathological full-epilogue case.

We should have an auto mode to detect which mode yields space savings.
It seems easy to detect when a diff you're building will be larger than the prologue itself and fallback to full epilogue in these cases.

[Dando18]

I like the auto mode best, but another option would be to add a mneme convert-epilogue command for doing this after the recording. Something like mneme convert-epilogue --format <bytes|diff|best> --db *.epilogue.*.mneme.