We had the legacy "memIndex" option to put the Camlistore index in
memory (the leveldb memdb) as an option in genconfig called
"memIndex", and we also had an option called "memoryIndex" for whether
to slurp the on-disk index to memory on start-up. Too confusing!
Instead, delete "memIndex" (since it's not the default anyway, now
that we have kv).
Then, also (the original point of this change): default the
memoryIndex option to true, so search works for people by default.
This option might go away in the future if it becomes the only required
way.
Also, document this.
Change-Id: Iddffa6e19adbf09c5aacd063aa44de362d90633b
Brad Fitzpatrick