If the shares archive table is used via config option, those shares are
included when calculating the users hashrate. This will help on very
short rounds where each round end deletes all shares and users will not
be able to track their hashrates. They will be very jumpy and change to
0 all the time.
Still missing this implementation for the hourly hashrate function to
ensure the graphs are always kept up-to-date.
Addresses #331
306162a1da