Reduce size of parents cache

[schomatis]

The expansions parents (the only ones saved in the cache, the base ones are generated on demand) are going to be increased from 8 to 16 (#857) which will have a considerable impact on memory consumption (when the cache is activated). The first measure to alleviate this increase is try to move the cache from memory to disk (#855). It is also desirable (if the previous strategy doesn't work, or at least as a compliment to it) to reduce the size of the cache as much a possible. Some suggestions are scattered throughout different issues, the main ones are:

1. Use 32 bit node indexes (instead of the current u64) when the sector size is small enough.
2. Do not include padding nodes.

[dignifiedquire]

parents cache is using u32s now