MODE_WRITE mapupdate: maybe increase epsilon to handle large batches of new servers better

[warner]

Part of the problem described in #546 is due to adding a large number of new servers. This inserts large gaps in the permuted server list, and the mapupdate code that handles MODE_WRITE may not look far enough to get past the gaps.

It might be a good idea to increase the "epsilon" used for MODE_WRITE from its current value of "k" (usually 3) to "N" (usually 10). As noted at the bottom of #546, if all the servers are writeable, this should be enough to make sure we don't need to send any shares to unqueried servers.