ComputeFstat: extract timestamps from (loaded) SFTs, not catalog

- Catalog may contain entries for SFTs which are not loaded,
  e.g. E@H narrow-band SFTs which fall outside the search band
- Timestamps are needed for fake SFT generation, however, so need
  to rearrange order of loading/generating SFTs so that the same
  timestamps are used.
- Downside is that, when loading and generating SFTs, 2xSFT memory
  will be needed (one for loaded, one for generated SFTs), since
  XLALCWMakeFakeMultiData cannot add to existing SFTs (see #1466)
- Refs #1520