Partial conversion / memory usage

[gforcada]

It would be useful to be able to ask to parse only X amount of objects so one can get some partial results and at the same time it would be useful to use this option with a memory profiler to get to understand why the memory usage just keeps growing and nothing is ever freed (in one Data.fs I have access to).

[icemac]

Version 0.2 has to new options:

• --start to start the analysis with a predefined OID.
• --limit to stop the analysis after a certain amount of seen OIDs.

Note that I split up the two functions previously united in the script bin/zodb-py3migrate into `bin/zodb-py3migrate-analyzeresp.bin/zodb-py3migrate-convert``.