A few nitpicks with recent getrawmempool updates.

This commit addresses a few nitpicks in the recent getrawmppol update
which populates the starting and current priority fields.

In particular:

- Move the new calcInputValueAge function before the function which
  invokes it so it is consistent with the rest of the mempool code
- Double space after periods for consistency
- Correct the comments for calcInputValueAge to indiciate that inputs
  which are in the the memory pool count as zero toward the value age
  rather than the incorrect claim that that the overal input value age is
  zero when one of them does
- Rename endingPriority to currentPriority to match the RPC field and its
  actual function
- Make the comment about using zero when input transactions can't be found
  for some reason more accurate since there can be (and frequently is)
  more than one input transaction