Move the entire fingerprint cache population into fprint.c

Move the entire fingerprint cache population into fprint.c

- Rename addFingerprints() to fpCachePopulate() and move into fprint.c.
  This doesn't really belong here as it requires fprint becoming aware
  of transactions and all, but at least these are all controlled API
  accesses unlike where in transaction.c this was messing with somebody
  elses data structures directly.
- Move the by-fingerprint creation to fpCachePopulate() so it gets
  lazily done as needed and copy the original hash-size heuristics
  back here.