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.

[zooko]

It's really bothering me that mutable file upload and download behavior is so finicky, buggy, inefficient, hard to understand, different from immutable file upload and download behavior, etc. So I'm putting a bunch of tickets into the "1.8" Milestone. I am not, however, at this time, volunteering to work on these tickets, so it might be a mistake to put them into the 1.8 Milestone, but I really hope that someone else will volunteer or that I will decide to do it myself. :-)