More efficient scheme allows us to increase the check frequency.

The code in crc-check.js and the back-end handler now provide an MD5
hash, so that the checker can transfer that each time instead of the
whole file every time.  This means a lot less data is being passed back
and forth, which means we can afford to increase the check frequency.
Use an md5 file on the server, CRC only as a fallback.

This now tries to download an MD5 has file from the server 1st, so that
the checker can transfer that each time instead of the whole file every
time.  This means a lot less data is being passed back and forth, which
means we can afford to increase the check frequency.