ENH+FIX: Only read necessary part of tag in raw reading

@agramfort this addresses the issue that we hit with the MEGSIM dataset, where all the data was contained within a single tag. It also sped up epoching non-preloaded raw data by ~30% in my tests. I know it touches some critical code, so we should think carefully about if this is the best implementation. (The underlying code changes are fairly straightforward, in any case). Note that I added an additional test to make sure that the reading is working correctly.